Optical waveguide, optical wiring component, optical waveguide module and electronic device

ABSTRACT

A core layer ( 13 ) of an optical waveguide ( 1 ) includes a plurality of core groups ( 140 ) disposed so as to mutually intersect on the same plane, each core group ( 140 ) being an assembly of a plurality of core portions ( 14 ), at least some of which are arranged in parallel, and side cladding portions ( 15 ) provided so as to adjoin the side surfaces of each core portion ( 14 ). A transverse cross-section of the optical waveguide ( 1 ) includes a high refractive index region (WH) in a position corresponding with each core portion ( 14 ) and having a relatively high refractive index, and a low refractive index region (WL) in a position corresponding with each side cladding portion ( 15 ) and having a lower refractive index than the high refractive index region (WH), and a refractive index distribution is formed in which the refractive index varies continuously across the entire distribution.

TECHNICAL FIELD

The present invention relates to an optical waveguide, an optical wiringcomponent, an optical waveguide module and an electronic device.

Priority is claimed on Japanese Patent Application No. 2012-040798,filed Feb. 27, 2012, the content of which is incorporated herein byreference.

BACKGROUND ART

Optical communication technology which uses optical carrier waves totransport data is now being developed, and in recent years, opticalwaveguides are becoming increasingly widespread as a means of guidingthese optical carrier waves from one point to another. These opticalwaveguides have a linear core portion and a cladding portion provided soas to cover the periphery of the core portion. The core portion iscomposed of a material that is essentially transparent to the light ofthe optical carrier waves, and the cladding portion is composed of amaterial having a lower refractive index than that of the core portion.

In an optical waveguide, light introduced from one end of the coreportion is transported to the other end while reflecting off theboundaries with the cladding portion. A light emitting element such as asemiconductor laser is disposed at the input side of the opticalwaveguide, and a light receiving element such as a photodiode isdisposed at the output side. The light input from the light emittingelement is transmitted through the optical waveguide and is received bythe light receiving element, and communication is conducted on the basisof a blinking pattern or intensity pattern of the received light.

The use of these types of optical waveguides in supercomputers andlarge-scale servers and the like is being investigated. Conventionalsupercomputers are constructed by installing a plurality of electricalcircuit boards mounted with semiconductor elements and electroniccomponents and the like in racks, and then electrically connecting theseelectrical circuit boards to one another. Investigations are beingconducted for such structures, for example, into substituting electricalconnections within individual electrical circuit boards, electricalconnections between electrical circuit boards and electrical connectionsbetween racks with optical connections using optical fibers. It isanticipated that these substitutions will enable greater volumes ofinformation transmission, increased speed, and reduced energyconsumption and the like, resulting in improved supercomputerperformance.

In order to achieve these optical connections, optical fiber sheets arebeing investigated in which a plurality of optical fibers are bundledtogether in an intersecting state, with connectors provided at the endsof the fibers (for example, see Patent Document 1).

In order to replace electrical wiring with this type of optical fibersheet, light receiving and emitting elements and connectors areinstalled on the electrical circuit board. Then, by linking theelectrical circuit board side connectors with the optical fiber ribbonside connectors, optical connections are achieved. Further, devices inwhich light receiving and emitting elements are installed on the side ofthe optical fiber sheet are also being investigated.

However, these optical fiber sheets are formed by sandwiching theintersecting portions of the optical fibers between film substrates.Accordingly, the optical fibers overlap at the optical fiberintersecting portions, meaning an increase in the sheet thickness atthese portions is unavoidable. Consequently, the sheets are difficult tobend during optical connection operations, and there is a possibilitythat the optical fibers may break when bent with excessive strength. Asa result, there are various restrictions associated with the wiringspace and the wiring operations.

Further, in consideration of resistance to transverse rupture and thelike, making the optical fibers finer is problematic. Accordingly, thespacing between the core portions of adjacent optical fibers cannot benarrowed more than conventional structures, meaning there is a limit topossible improvements in the wiring density.

BACKGROUND ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Unexamined Patent Application, First    Publication No. 2004-126310

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide an optical waveguide inwhich core portions can be intersected without an accompanying increasein thickness and the core portions can be formed at high density, anoptical wiring component and an optical waveguide module which includethe optical waveguide and can simplify optical wiring and contribute tospace saving, and an electronic device which can be readilyminiaturized.

Means to Solve the Problems

The above object is achieved by the following aspects (1) to (16) of thepresent invention.

(1) An optical waveguide having a plurality of core groups disposed soas to mutually intersect on the same plane, each core group being anassembly of a plurality of core portions, wherein the core portions areat least arranged partly paralleled, and side cladding portions providedon both side surfaces of each core portion so as to adjoin the coreportion, wherein

a transverse cross-section of the optical waveguide includes a highrefractive index region in a position corresponding with each of thecore portions and having a relatively high refractive index, and a lowrefractive index region in a position corresponding with each of theside cladding portions and having a lower refractive index than the highrefractive index region, and

a refractive index distribution in which the refractive index variescontinuously, within at least a portion of the distribution or acrossthe entire distribution, is formed by the regions.

(2) The optical waveguide according to (1) above, wherein the refractiveindex distribution is formed in correspondence with the concentration ofa refractive index modifier, which is dispersed in a polymer layer andhas a different refractive index from the polymer.

(3) The optical waveguide according to (2) above, wherein the refractiveindex distribution is formed by irradiating light partially onto thepolymer layer, which is composed of a material containing a polymer anda photopolymerizable monomer having a different refractive index fromthe polymer dispersed within the polymer, thereby causing migration anduneven distribution of the photopolymerizable monomer, and generatingvariation in the refractive index within the layer.

(4) The optical waveguide according to any one of (1) to (3) above,wherein when two of the core portions mutually intersect and the angleof intersection of the optical axes of the core portions is 90°, thetransmission loss in the intersection portion between the two coreportions is not more than 0.02 dB.

(5) The optical waveguide according to any one of (1) to (4) above,wherein the width of the core portions is from 10 to 200 μm.

(6) An optical wiring component, having the optical waveguide accordingto any one of (1) to (5) above, and connectors provided at the ends ofthe core groups of the optical waveguide.

(7) The optical wiring component according to (6) above, wherein theoptical waveguide has an optical path conversion portion, which isformed partway along the core portions or on an extended line thereof,and converts the optical paths of the core portions.

(8) An optical waveguide module, having the optical waveguide accordingto any one of (1) to (5) above, and light receiving and emittingelements which are provided on one surface of the optical waveguide andare optically connected to the core portions.

(9) An electronic device containing the optical waveguide according toany one of (1) to (5) above.

(10) The optical waveguide according to (1) above, wherein a refractiveindex distribution W in a width direction of a transverse cross-sectionof the optical waveguide has a region having at least two local minimumvalues, at least one first local maximum value, and at least two secondlocal maximum values smaller than the first local maximum value, thesevalues being arranged in a sequence composed of second local maximumvalue, local minimum value, first local maximum value, local minimumvalue and second local maximum value, and

within this region, a region sandwiched by the two local minimum valuesso as to include the first local maximum value corresponds with the coreportion, and regions from each local minimum value to the second localmaximum value correspond with the cladding portions,

each local minimum value has a value less than the average refractiveindex in the cladding portions, and

the refractive index varies continuously across the entire refractiveindex distribution W.

(11) The optical waveguide according to (1) above, wherein

the core portions and the side cladding portions form a core layer,

cladding layers are laminated to both surfaces of the core layer,

a refractive index distribution W in a width direction of a transversecross-section of the core layer has a region having at least two localminimum values, at least one first local maximum value, and at least twosecond local maximum values smaller than the first local maximum value,these values being arranged in a sequence composed of second localmaximum value, local minimum value, first local maximum value, localminimum value and second local maximum value,

within this region, a region sandwiched by the two local minimum valuesso as to include the first local maximum value corresponds with the coreportion, and regions from each local minimum value to the second localmaximum value correspond with the side cladding portions,

each local minimum value is less than the average refractive index inthe cladding portions,

the refractive index varies continuously across the entire refractiveindex distribution, and

in a refractive index distribution T in a thickness direction of atransverse cross-section of the optical waveguide, the refractive indexis substantially constant in a region corresponding with the coreportion and regions corresponding with the cladding layers, and therefractive index varies discontinuously at the interfaces between thecore portion and the cladding layers.

(12) The optical waveguide according to (1) or (10) above, wherein

the core portions and the side cladding portions form a core layer,

cladding layers are laminated to both surfaces of the core layer,

in a refractive index distribution W in a width direction of atransverse cross-section of the optical waveguide, the refractive indexis substantially constant in the low refractive index regions, and

in a refractive index distribution T in a thickness direction of atransverse cross-section of the optical waveguide, the refractive indexis substantially constant in a region corresponding with the core layerand regions corresponding with the cladding layers, and the refractiveindex varies discontinuously at the interfaces between the core portionand the cladding layers.

(13) The optical waveguide according to (1) or (10) above, wherein

the core portions and the side cladding portions form a core layer,

cladding layers are laminated to both surfaces of the core layer,

in a refractive index distribution W in a width direction of atransverse cross-section of the optical waveguide, the refractive indexis substantially constant in the low refractive index regions, and

a refractive index distribution T in a thickness direction of atransverse cross-section of the optical waveguide includes a regioncorresponding with the core portion of the core layer, and regionscorresponding with the cladding layers,

the refractive index varies continuously in the region correspondingwith the core portion of the core layer,

the refractive index is substantially constant in the regionscorresponding with the cladding layers, and

the refractive index varies discontinuously at the interfaces betweenthe core portion and the cladding layers.

(14) The optical waveguide according to (1) or (10) above, wherein

the core portions and the side cladding portions form a core layer,

cladding layers are laminated to both surfaces of the core layer,

a refractive index distribution T in a thickness direction of atransverse cross-section of the optical waveguide has a local maximumvalue, first portions in which the refractive index decreasescontinuously from the position of the local maximum value toward thecladding layers, and second portions, positioned on the opticalwaveguide upper and lower surface sides of the first portions, in whichthe refractive index is substantially constant, and

a region corresponding with the local maximum value and the firstportions represents the core portion, and regions corresponding with thesecond portions represent the cladding layers.

(15) The optical waveguide according to (1) above, wherein

the core portions and the side cladding portions form a core layer,

cladding layers are laminated to both surfaces of the core layer,

a refractive index distribution T in a thickness direction of atransverse cross-section of the optical waveguide has a region having atleast two local minimum values, at least one first local maximum value,and at least two second local maximum values smaller than the firstlocal maximum value, these values being arranged in a sequence composedof second local maximum value, local minimum value, first local maximumvalue, local minimum value and second local maximum value,

within this region, a region sandwiched by the two local minimum valuesso as to include the first local maximum value corresponds with the corelayer, and regions from each local minimum value to the second localmaximum value correspond with the cladding layers,

each local minimum value has a value less than the average refractiveindex in the cladding layers, and

the refractive index varies continuously across the entire refractiveindex distribution T.

(16) The optical waveguide according to any one of (1) to (18) above,satisfying at least one of features (i) to (v) described below:

(i) the thickness of the core layer is about 1 to 200 μm, preferably 5to 100 μm, and more preferably 10 to 50 μm,

(ii) the average width of the side cladding portions is within a rangefrom 5 to 250 μm,

(iii) a ratio between the average width of the core portions and theaverage width of the side cladding portions is within a range from 0.1to 10,

(iv) the average thickness of the cladding layers is from 0.01 to 7times the average thickness of the core layer, and

(v) in a transverse cross-section of the core layer, if the width of aportion of the core layer in which the refractive index is continuouslyequal to or greater than the average refractive index of the sidecladding portions is denoted a, and the width of a portion of the corelayer in which the refractive index is continuously less than theaverage refractive index of the side cladding portions is denoted b,then b is within a range from 0.01a to 1.2a.

(17) The optical waveguide according to any one of (10) to (15) above,satisfying at least one of features (i) to (vi) described below:

(i) in the refractive index distribution in the transverse direction ofa transverse cross-section, a difference between the average refractiveindex of the local minimum values and the average refractive index ofthe side cladding portions is from 3 to 80% of the difference betweenthe average refractive index of the local minimum values and the averagerefractive index of the first local maximum value,

(ii) in the refractive index distribution in the transverse direction ofa transverse cross-section, a difference between the average refractiveindex of the local minimum values and the average refractive index ofthe second local maximum values is from 6 to 90% of the differencebetween the average refractive index of the local minimum values and theaverage refractive index of the first local maximum value,

(iii) in the refractive index distribution in the width direction of atransverse cross-section, a refractive index difference between theaverage refractive index of the local minimum values and the averagerefractive index of the first local maximum value is from 0.005 to 0.07,

(iv) in the refractive index distribution in a thickness direction of atransverse cross-section, if the width of a portion in which therefractive index is equal to or greater than the average refractiveindex of the cladding layers is denoted a, and the width of a portion inwhich the refractive index is less than the average refractive index ofthe cladding layers is denoted b, then b is within a range from 0.01a to1.2a,

(v) in the refractive index distribution in a thickness direction of atransverse cross-section, a difference between the average refractiveindex of the local minimum values and the average refractive index ofthe cladding layers is from 3 to 80% of the difference between theaverage refractive index of the local minimum values and the first localmaximum value within the core portion, and

(vi) in the refractive index distribution in a thickness direction of atransverse cross-section, the difference between the average refractiveindex of the local minimum values and the average refractive index ofthe first local maximum value within the core portion is from 0.005 to0.07.

The high refractive index region is preferably composed of a peakportion indicated by the local maximum value, and two tailing portionsin which the refractive index decreases continuously from the localmaximum value toward both sides, and the difference between the localmaximum value and the average refractive index of the low refractiveindex regions is preferably from 0.005 to 0.07.

The above refractive index distribution preferably includes localminimum values, which are positioned at the interface portions betweenthe low refractive index regions and the high refractive index region,and have a lower refractive index than the average refractive index ofthe low refractive index regions.

If necessary, the refractive index of the low refractive index regionsmay be substantially constant.

Effects of the Invention

The present invention enables an optical waveguide to be obtained inwhich core portions can be intersected without an accompanying increasein thickness, and the core portions can be formed at high density.

Further, the present invention also enables an optical wiring componentand an optical waveguide module to be obtained which include the aboveoptical waveguide, and can simplify optical wiring and contribute tospace saving.

Moreover, the present invention also enables an electronic device to beobtained which includes the above optical waveguide and can be readilyminiaturized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating an example of an optical wiringcomponent of the present invention.

FIG. 2 is a perspective view illustrating a first embodiment of anoptical waveguide of the present invention.

FIG. 3 is a diagram schematically illustrating one example of arefractive index distribution W (GI type) for the cross-section alongthe line X-X of the optical waveguide illustrated in FIG. 2.

FIG. 4 is a diagram schematically illustrating another example of arefractive index distribution W (W type) for the cross-section along theline X-X of the optical waveguide illustrated in FIG. 2.

FIG. 5 is a diagram illustrating one example of the intensitydistribution of the output light when light is incident upon one of thetwo core portions of an optical waveguide having the refractive indexdistribution illustrated in FIG. 4.

FIG. 6A is a diagram illustrating a portion of the cross-section alongthe line X-X of the optical waveguide illustrated in FIG. 2, and alsoschematically illustrating one example of a refractive indexdistribution T (W type) in the thickness direction of that portion.

FIG. 6B is a diagram illustrating a portion of the cross-section alongthe line X-X of the optical waveguide illustrated in FIG. 2, and alsoschematically illustrating one example of a refractive indexdistribution T (GI type) in the thickness direction of that portion.

FIG. 7A is a top view of the optical waveguide illustrated in FIG. 2,with the cover film and the cladding layer omitted.

FIG. 7B is a diagram illustrating the vicinity of an intersectionportion of the optical waveguide illustrated in FIG. 7A, and therefractive index distributions in the thickness and width directions.

FIG. 8A is partial enlargement illustrating another example of anintersection portion and the vicinity thereof for the core layerillustrated in FIG. 7A.

FIG. 8B is partial enlargement illustrating yet another example of anintersection portion and the vicinity thereof for the core layerillustrated in FIG. 7A.

FIG. 8C is partial enlargement illustrating yet another example of anintersection portion and the vicinity thereof for the core layerillustrated in FIG. 7A.

FIG. 8D is partial enlargement illustrating yet another example of anintersection portion and the vicinity thereof for the core layerillustrated in FIG. 7A.

FIG. 9 is a perspective view illustrating a second embodiment of anoptical waveguide of the present invention.

FIG. 10 is a diagram illustrating a portion of the cross-section alongthe line Y-Y shown in FIG. 9, and also schematically illustrating oneexample of a refractive index distribution T (W type) in the thicknessdirection of that portion.

FIG. 11 is a diagram illustrating one example of the intensitydistribution of the output light at the output end surface when light isincident upon one of the two core portions of the optical waveguideillustrated in FIG. 9.

FIG. 12 is a plan view illustrating another example of an optical wiringcomponent of the present invention.

FIG. 13 is a diagram describing an example of a method of producing theoptical waveguide illustrated in FIG. 2.

FIG. 14 is a diagram describing an example of a method of producing theoptical waveguide illustrated in FIG. 2.

FIG. 15 is a diagram describing an example of a method of producing theoptical waveguide illustrated in FIG. 2.

FIG. 16 is a diagram describing the generation of a refractive indexdifference between an irradiated region and a non-irradiated region inthe laminated structure illustrated in FIG. 14.

FIG. 17A is a diagram illustrating the refractive index distribution (Wtype) in the thickness direction of the laminated structure of thecladding layers and the core layer of FIG. 14, prior to irradiation withactive radiation.

FIG. 17B is a diagram illustrating the change in the refractive indexdistribution in the thickness direction of the laminated structure ofthe cladding layers and the core layer of FIG. 14 following irradiationwith active radiation.

FIG. 17C is a diagram illustrating the refractive index distribution inthe thickness direction in the optical waveguide of FIG. 13, prior toirradiation with active radiation, when the uppermost layer and thelowermost layer produced from an optical waveguide-forming composition901 are not provided.

FIG. 18 is a graph illustrating the relationship between the number ofintersections and the relative loss for optical waveguides of thepresent invention and the conventional technology, at a core layercrossing angle of 90°.

FIG. 19 is a graph illustrating the relationship between the number ofintersections and the relative loss for optical waveguides of thepresent invention, at a core layer crossing angle of 60°.

FIG. 20 is a graph illustrating the relationship between the number ofintersections and the relative loss for optical waveguides of thepresent invention, at a core layer crossing angle of 30°.

FIG. 21 is a graph illustrating the results of measuring the refractiveindex distribution for the core portion and the side cladding portionspositioned on both sides thereof, in a transverse cross-section of anoptical waveguide (transverse W type and longitudinal SI type) of thepresent invention.

FIG. 22 is a three dimensional refractive index distributionillustrating the refractive index distributions in the width directionand the longitudinal direction of an optical waveguide (transverse Wtype and longitudinal GI type).

FIG. 23 is an interference fringe photograph, obtained using aninterference microscope, of an optical waveguide fragment obtained froma width direction cross-section of an optical waveguide of the presentinvention (containing three core portions).

BEST MODE FOR CARRYING OUT THE INVENTION

The optical waveguide, optical wiring component, optical waveguidemodule and electronic device of the present invention are describedbelow in further detail, based on preferred embodiments illustrated inthe appended drawings.

The present invention is not limited solely to these examples. Variousmodifications, omissions and/or additions can be made as desired withinthe scope of the present invention. The number, position and size ofdevices may also be modified as required.

<Optical Wiring Component>

First the optical wiring component of the present invention and theoptical waveguide of the present invention included therein aredescribed.

FIG. 1 is a plan view (shown through the cladding layer) illustrating anembodiment of the optical wiring component of the present invention.

The optical wiring component 10 illustrated in FIG. 1 has an opticalwaveguide 1 and connectors 101 provided at the ends thereof.

The optical waveguide 1 has a rectangular shape in plan view, and aplurality of core groups 140, each core group being an assembly of aplurality of core portions 14 (in FIG. 1, a single core group includesfour core portions 14 aligned in parallel), is arranged inside theoptical waveguide in a desired pattern (FIG. 1 includes four core groups140). This plurality of core groups 140 is disposed so as to mutuallyintersect within the same plane, and the two ends of the core groups areexposed at two opposing edges among the four edges of the opticalwaveguide 1. According to this type of optical waveguide 1, mutuallyintersecting complex and high-density signal paths can be constructedwithout an accompanying increase in the thickness of the waveguide. As aresult, an optical wiring component 10 can be obtained which can beeasily bent, and which enables wiring operations to be performed withrelative ease, even in confined wiring spaces. The core portions 14illustrated in FIG. 1 are each formed so as to trace a smooth curve whenviewed in plan view. By using this type of configuration, attenuation ofthe transmitted light can be suppressed, and any deterioration in thetransmission efficiency can be suppressed.

Further, the optical wiring component 10 also has the connectors 101provided at the ends of the core portions 14. The optical waveguide 10is formed so that the core portions of the optical waveguide 1 and otheroptical components can be optically connected via these connectors 101.In FIG. 1, the connectors 101 are provided on two opposing edges of theoptical waveguide 1, but the positioning of the connectors 101 is notlimited to this particular configuration.

Each of the portions of the optical wiring component 10 is describedbelow in further detail.

(Optical Waveguide)

<<First Embodiment>>

First is a description of a first embodiment of the optical waveguide ofthe present invention.

The optical waveguide 1 of the present invention is a sheet-like memberhaving core portions 14 and cladding portions 15, and functions as aseries of optical wires which transmit optical signals from one end tothe other. There are no particular limitations on the plan view shape ofthe optical waveguide 1, and a triangular shape, square shape, polygonalshape having 5 or more sides, or a circular shape or the like are alsopossible.

FIG. 2 is a perspective view illustrating the first embodiment of theoptical waveguide of the present invention (with a portion cut away, andillustrated as partially transparent).

The optical waveguide 1 illustrated in FIG. 2 has a support film 2, andthree layers including a cladding layer 11, a core layer 13 and acladding layer 12 laminated in that order on the support film 2.Further, the optical waveguide 1 illustrated in FIG. 2 is sandwichedbetween the support film 2 from below and a cover film 3 from above.

Further, the core layer 13 is formed from two parallel core portions 14(a first core portion 141 and a third core portion 142), a single coreportion 14 (a second core portion 145) which intersects each of the twoparallel core portions 14 at right angles, and side cladding portions 15(a first side cladding portion 151, a second side cladding portion 152and a third side cladding portion 153) which are adjacent to and adjointhese core portions 14.

The core layer 13 of the optical waveguide 1 illustrated in FIG. 2 has arefractive index distribution W which exhibits variation in therefractive index in the width direction. The refractive indexdistribution W includes high refractive index regions each including alocal maximum value, and low refractive index regions having a lowerrefractive index than that of the high refractive index regions, and therefractive index varies continuously within at least a portion of thedistribution or across the entire distribution. With this type ofrefractive index distribution W, the core portions 14 are positionedcorresponding with the high refractive index regions, and the sidecladding portions 15 are positioned corresponding with the lowrefractive index regions. The aforementioned expression “within at leasta portion” describes a distribution (GI type) in which a region having aconstant refractive index exists within the low refractive indexregions. Further, the expression “across the entire distribution”describes a distribution (W type) in which the refractive index variescontinuously even in the low refractive index regions.

This refractive index distribution W is formed by partially irradiatinglight onto a layer composed of a material containing a polymer and aphotopolymerizable monomer having a different refractive index from thepolymer, the photopolymerizable monomer being dispersed within thepolymer, thereby causing migration and uneven distribution of thephotopolymerizable monomer and generating variation in the refractiveindex within the layer. Because the distribution is formed on theseprinciples, the refractive index in the refractive index distribution Wvaries continuously.

The optical waveguide 1 having the types of features described above cantransmit incident light while confining the light to the regions of highrefractive index. Particularly in the case of the optical waveguide 1having the refractive index distribution W, because the change in therefractive index is continuous, the incident light is transmittedconcentrated in the vicinity of the local maximum value for therefractive index in the refractive index distribution W. As a result,transmission loss and rounding of the pulse signal can be suppressed,and even if a large volume of optical signals are input, highly reliableoptical communication can be achieved. Further, even when the pluralityof core portions 14 mutually intersect within the same plane, opticalsignal interference is suppressed. Furthermore, the optical waveguide 1can be formed by a step of simply selecting and irradiating a lightirradiation region. Accordingly, even if a plurality of core portions 14is formed to generate multiple channels, the spacing between coreportions 14 is reduced to increase the density, or a plurality of coreportions 14 intersect at a single point, the shape can be produced inaccordance with the design, and high-quality optical communication canbe performed.

Furthermore, the refractive index distribution W is a distribution inwhich, as described above, the refractive index varies continuously dueto the migration and uneven distribution of the photopolymerizablemonomer. As a result, the core layer 13 has no clear structuralinterfaces between the core portions 14 and the side cladding portions15 formed within the core layer 13. Accordingly, peeling or crackingbetween the core portions 14 and the side cladding portions 15 isunlikely, making the optical waveguide 1 a highly reliable product.

Preferred examples of each of the portions of the optical waveguide 1are described below in detail.

(Core Layer)

In the core layer 13, if each core portion 14 is cut in a planeorthogonal to the longitudinal direction, then as described above, arefractive index distribution W in which the refractive index varies inthe width direction can be formed within the cut cross-section (thefirst transverse cross-section).

In the present invention, when the expression “transverse cross-sectionof the optical waveguide” is used, this expression may be thought of asdescribing a cross-section orthogonal to the longitudinal direction ofthe core portion.

FIG. 3(a) illustrates a cross-sectional view along the line X-X of theoptical waveguide illustrated in FIG. 2. FIG. 3(b) is a diagramschematically illustrating one example of the refractive indexdistribution W (GI type) along a centerline C1 which penetrates alongthe center in the thickness direction of the core layer 13 in thecross-sectional view, with the distance in the width direction shownalong the horizontal axis, and the refractive index shown along thevertical axis.

As illustrated in FIG. 3(b), the refractive index distribution W isprovided with a distribution which corresponds with the position of eachcore portion 14 and the position of each side cladding portion 15 (awidth direction distribution, hereafter referred to as the refractiveindex distribution W). The refractive index distribution W has highrefractive index regions WH of relatively high refractive index, whichinclude a local maximum value Wm provided corresponding with theposition of each core portion 14, and two tailing portions in which therefractive index decreases continuously toward both sides from thislocal maximum value Wm. Further, the refractive index distribution Walso has low refractive index regions WL of relatively low refractiveindex provided corresponding with the position of each of the sidecladding portions 15. In the high refractive index regions WH, on bothsides of the local maximum value Wm, the refractive index traces a curvewhich continuously decreases toward the respective adjacent lowrefractive index region WL. In other words, in the high refractive indexregions WH, the refractive index is distributed with the peak at thelocal maximum value Wm, and the refractive index then decreasing insmooth tails toward both sides. On the other hand, in the low refractiveindex regions WL, the refractive index is lower than the refractiveindex of the high refractive index regions WH, and is a substantiallyconstant value.

Furthermore, the plurality of local maximum values Wm which exist withinthe refractive index distribution W preferably have the same value, butmay vary slightly. In such cases, the variation is preferably not morethan 10% of the average value of the plurality of local maximum valuesWm.

Each of the two parallel core portions 14 (the first core portion 141and the third core portion 142) has an elongated linear shape. In thetype of refractive index distribution W described above, substantiallythe same distribution is maintained along the entire longitudinaldirection of these core portions 14.

On the other hand, the type of refractive index distribution W describedabove is also formed in the core portion 14 (the second core portion145) which intersects the above core portions (the first core portion141 and the third core portion 142). In other words, substantially thesame distribution is maintained along the entire longitudinal directionof this core portion (the second core portion 145).

With the type of refractive index distribution W described above, thetwo elongated core portions 14, the core portion 14 which intersectsthese core portions 14, and the side cladding portions 15 which adjointhe side surfaces of these core portions 14 are formed within the corelayer 13 illustrated in FIG. 2.

More specifically, within the core layer 13 illustrated in FIG. 2 areprovided the two parallel core portions 141 and 142 (the first coreportion and the third core portion), the core portion 145 (the secondcore portion) that intersects these core portions, and the first sidecladding portion 151, the second side cladding portion 152 and the thirdside cladding portion 153 which are provided in the regions outsidethese core portions. Each of the core portions 141, 142 and 145 existsin a state surrounded by the side cladding portions 151, 152 and 153,and the upper and lower cladding layers 11 and 12. The refractiveindices of the core portions 141, 142 and 145 are higher than therefractive indices of the side cladding portions 151, 152 and 153.Accordingly, light can be confined in the width direction of each of thecore portions 141, 142 and 145. Each core portion 14 illustrated in FIG.1 is represented by a region of dense dots, and each side claddingportion 15 is represented by a region of sparse dots.

Further, in the optical waveguide 1, light incident at one end of a coreportion 14 is transmitted to the other end while also being confinedwithin the thickness direction of the core portion 14, and can beextracted from the other end of the core portion 14.

In the core portions illustrated in FIG. 1, the transversecross-sectional shape of the portions has a quadrangular (rectangular)shape such as a square or a rectangle. However, there are no particularlimitations on the shape of the transverse cross-section, and a circularshape such as a perfect circle, ellipse or oval, or a polygonal shapesuch as a triangle, pentagon or hexagon may also be used. When thetransverse cross-sectional shape of the core portions 14 is rectangular,core portions of stable quality can be produced with good efficiency. Onthe other hand, when the transverse cross-sectional shape of the coreportions 14 is circular, the transmission efficiency of the coreportions 14 improves, the convergence of the transmitted light improves,and the optical coupling efficiency with other optical components isenhanced.

There are no particular limitations on the width of the core portions 14and the height of the core portions (the thickness of the core layer13), but each is preferably about 1 to 200 μm, more preferably about 5to 100 μm, and still more preferably about 10 to 70 μm. This enablescrosstalk in the optical waveguide 1 to be better suppressed. There arealso no particular limitations on the thickness of the core layer, butthe thickness is preferably about 1 to 200 μm, more preferably about 5to 100 μm, and still more preferably about 10 to 70 μm.

In the refractive index distribution W, in at least a portion of thedistribution or across the entire distribution, the refractive indexvaries continuously and traces out a curve. As a result of this feature,the light confinement effect within the core portions 14 is enhancedcompared with an optical waveguide having a so-called step index type(SI type) refractive index distribution in which the refractive indexchanges in a stepwise manner, and therefore further reductions intransmission loss can be achieved.

Moreover, in the refractive index distribution W described above, therefractive index varies continuously in the region mentioned above, andincludes a local maximum value. Accordingly, because the speed of thelight is inversely proportional to the refractive index, the speed ofthe light increases with increasing distance from the center, andtransmission time differences between optical paths are unlikely. As aresult, the transmitted waveform is less likely to deteriorate. Forexample, even if the transmitted light contains a pulse signal, roundingof the pulse signal (broadening of the pulse signal) can be suppressed.In addition, interference between transmitted light signals at theintersection portions is also inhibited. As a result, an opticalwaveguide 1 which can improve the quality of optical communication canbe obtained.

In the present invention, the expression that the refractive indexvaries continuously within the refractive index distribution W describesa state in which the curve of the refractive index distribution W isrounded at each portion of the distribution, with this curve beingdifferentiable.

Further, within the refractive index distribution W, as illustrated inFIG. 3, the local maximum values Wm are positioned within the coreportions 141 and 142. Within these core portions 141 and 142, the localmaximum values Wm are preferably positioned in the central portion ofthe width of each core portion. This feature increases the probabilitythat, within each core portion 141 and 142, the transmitted light willbe concentrated in the central portion of the width of the core portion,resulting in a relative reduction in the probability that light willleak into the side cladding portions 151, 152 and 153. As a result,transmission loss in the core portions 141 and 142 can be furtherreduced.

The central portion of the width of the core portion 141 describes theregion from the center of the refractive index distribution WH to adistance 30%, and preferably 15%, of the width of the refractive indexdistribution WH on both sides of the center.

Further, the difference between the local maximum value Wm and theaverage refractive index in the low refractive index region WL ispreferably as large as possible. The difference can be selected asrequired, but for example, is preferably from about 0.005 to 0.07, morepreferably from about 0.007 to 0.05, and still more preferably fromabout 0.01 to 0.03. As a result of this feature, light can be reliablyconfined within the core portions 141 and 142. In other words, if therefractive index difference is lower than the above lower limit, thenthere is a possibility that light may leak from the core portions 141and 142. On the other hand, if the refractive index difference exceedsthe above upper limit, then not only can no further improvement in thelight confinement effect be expected, but the production of the opticalwaveguide 1 may sometimes become more difficult.

Furthermore, as illustrated in FIG. 3(b), when the position (distance)across the transverse cross-section of the core layer 13 is shown alongthe horizontal axis and the refractive index is shown along the verticalaxis, the refractive index distribution W in each of the core portions141 and 142 preferably has a substantially inverted U-shape, that is aconvex form, in the vicinity of the local maximum value Wm. As a resultof this shape, the light confinement effect in the core portions 141 and142 becomes even more marked.

On the other hand, the deviation of the refractive index from theaverage refractive index in the low refractive index regions WL ispreferably not more than 5% of the average refractive index. Thisenables the low refractive index regions WL to function reliably as theside cladding portions 15.

By using the refractive index distribution W described above, a varietyof effects can be obtained, including a reduction in transmission loss,a reduction in rounding of pulse signals, suppression of crosstalk, andsuppression of interference at the intersection portions. Further, theinventors of the present invention discovered that these effects areaffected significantly by the average width WCL of the side claddingportions, and/or the ratio between the average width WCO of the coreportions and the average width WCL of the side cladding portions. Theinventors found that when these factors satisfied prescribed ranges, theeffects mentioned above became more marked and reliable.

In the present invention, the ratio between the average width WCO of thecore portions 14 and the average width WCL of the side cladding portions15 (namely, WCO/WCL) is preferably within a range from 0.1 to 10. Byoptimizing the ratio between the widths of the core portions 14 and theside cladding portions 15, each of the effects described above can beenhanced. If WCO/WCL is less than the above lower limit, then there is apossibility that the average width of the core portions 14 may becometoo narrow, and therefore although a reduction in crosstalk can beachieved, transmission loss tends to increase, and miniaturization ofthe optical waveguide 1 may be hindered. Further, if WCO/WCL exceeds theabove upper limit, then there is a possibility that the average width ofthe side cladding portions 15 may become too narrow, resulting inincreased crosstalk, and the average width of the core portions 14 maybecome too wide, meaning there is a chance of increased rounding ofpulse signals.

The value of WCO/WCL is more preferably about 0.1 to 5, and still morepreferably about 0.2 to 4.

The average width WCO of the core portions 14 and the average width WCLof the side cladding portions 15 may each be selected as appropriate. Inaddition to the condition described above, preferred ranges for thesewidths have a lower limit which may be at least 0.1 μm, at least 1 μm,at least 5 μm, or at least 10 μm. Alternatively, the lower limit may beat least 30 μm, or at least 50 μm. The upper limit may be not more than5 mm, not more than 1 mm, not more than 0.5 mm, or not more than 0.2 μm.

Further, the distance between the local maximum values Wm may beselected as required. A preferred range for this distance has a lowerlimit that is preferably at least 10 μm, more preferably at least 20 μm,and still more preferably 30 μm or greater. The lower limit may also be50 μm or greater. Further, the upper limit is preferably not more than800 μm, more preferably not more than 500 μm, still more preferably notmore than 400 μm, and most preferably 300 μm or less. If required, theupper limit may be not more than 200 μm, or 100 μm or less.

The length of the core portions can be selected as required. Forexample, the length may be from 1 to 500 cm, from 2 to 200 cm, or from10 to 100 cm.

Moreover, in the present invention, independently from the WCO/WCLratio, or in addition to the ratio, the average width WCL of the sidecladding portions 15 is preferably within a range from 5 to 250 μm. Thisfeature enables each of the effects described above to be furtherenhanced. If WCL is less than the above lower limit, then the averagewidth of the side cladding portions 15 may become too narrow, andtherefore there are possibilities of increased rounding of pulse signalsand increased crosstalk. Further, if WCL exceeds the above upper limit,then the shape of the refractive index distribution W cannot beoptimized, and there is a possibility that transmission loss mayincrease. Moreover, there is a possibility that miniaturization of theoptical waveguide 1 may become more difficult.

The aforementioned WCL is more preferably within a range from 10 to 200μm, still more preferably within a range from 10 to 120 μm, andparticularly preferably within a range from 10 to 60 μm.

The average width WCO of the core portions 14 is also more preferablywithin a range from 10 to 200 μm, still more preferably within a rangefrom 10 to 120 μm, and particularly preferably within a range from 10 to60 μm.

Furthermore, in the present invention, the refractive index distributionW may include a flat portion in which the refractive index undergoesessentially no change in the vicinity of each local maximum value Wm.Even in this case, the optical waveguide of the present inventionexhibits the types of actions and effects described above. Here, theflat portion in which the refractive index undergoes essentially nochange describes a region in which the fluctuation in the refractiveindex is less than 0.001, and the refractive index decreases in acontinuous manner from both sides of the region.

There are no particular limitations on the length of this flat portion,but the length is preferably not more than 100 μm, more preferably 20 μmor less, and still more preferably 10 μm or less.

Furthermore, in the aforementioned tailing portions, the rate of changein the refractive index is preferably about 0.001 to 0.035 [/10 μm], andmore preferably about 0.002 to 0.030 [/10 μm]. Provided the rate ofchange in the refractive index satisfies the above range, the effectssuch as the reduction in the transmission loss in each of the coreportions 14, the reduction in the rounding of pulse signals, thesuppression of crosstalk, and the suppression of interference at theintersection portions are further enhanced.

Furthermore, the present embodiment described a case in which the corelayer 13 had three core portions 14. However, the number of coreportions 14 is not particularly limited, and may be selected as desired.For example, either 2 core portions, or 4 or more core portions may alsobe used. In these cases, in a similar manner to above, the refractiveindex distribution W has a distribution having a high refractive indexregion WH corresponding with each core portion 14, with a low refractiveindex region WL existing between high refractive index regions WH.

The constituent material (main material) of the type of core layer 13described above may be any material in which the refractive indexdifference described above occurs. Examples of materials that can beused include various resin materials, including acrylic-based resins,methacrylic-based resins, cyclic ether-based resins such as epoxy-basedresins and oxetane-based resins, polyimides, polybenzoxazoles,polysilanes, polysilazanes, silicone-based resins, fluororesins andpolyolefin-based resins such as norbornene-based resins, and glassmaterials. The resin material may be a composite material containing acombination of materials having different compositions.

Further, among these, the use of one or more material selected from thegroup consisting of (meth)acrylic-based resins, epoxy-based resins andpolyolefin-based resins is preferable. These resin materials exhibithigh light transmittance, and therefore yield an optical waveguide 1having particularly low transmission loss.

The refractive index distribution W may also include a local minimumvalue between the high refractive index region WH and the low refractiveindex region WL (the interface portion). By using this type ofconfiguration, the function of the waveguide in transmitting the lightwhile confined within the high refractive index region is enhanced, andtransmission loss and pulse signal rounding can be suppressed toparticularly low levels.

Furthermore, the low refractive index region WL preferably includes alocal maximum value (referred to as a “second local maximum value”)which is smaller than the local maximum value (referred to as the “firstlocal maximum value”) included in the high refractive index region WH.By including this type of second local maximum value in the lowrefractive index region WL, crosstalk between adjacent core portions inthe width direction is suppressed. As a result, even if a plurality ofcore portions is formed within the core layer 13 to generate multiplechannels, and the spacing between core portions is reduced to increasethe density, the optical waveguide 1 of the present invention can stillmaintain high quality optical communication. Moreover, even if aplurality of the core portions 14 mutually intersect within the sameplane, interference of the optical signals is suppressed.

FIG. 4(a) is a cross-sectional view along the line X-X of FIG. 2. FIG.4(b) is a diagram schematically illustrating another example of therefractive index distribution W along the centerline C1 which penetratesalong the center in the thickness direction of the core layer 13. Unlessspecified otherwise, this example may have the same conditions as theexample described above.

The refractive index distribution W illustrated in FIG. 4(b) has fourlocal minimum values Ws1, Ws2, Ws3 and Ws4, and five local maximumvalues Wm1, Wm2, Wm3, Wm4 and Wm5. Further, within the five localmaximum values there exist local maximum values (first local maximumvalues) Wm2 and Wm4 with relatively large refractive indices, and localmaximum values (second local maximum values) Wm1, Wm3 and Wm5 withrelatively small refractive indices.

Of these, the local maximum value Wm2 and the local maximum value Wm4exist between the local minimum value Ws1 and the local minimum valueWs2, and the local minimum value Ws3 and the local minimum value Ws4respectively.

In the optical waveguide 1 illustrated in FIG. 4, the local maximumvalue Wm2 having a relatively large refractive index is positionedbetween the local minimum value Ws1 and the local minimum value Ws2, andthis region having a relatively large refractive index becomes a coreportion 14. Similarly, the local maximum value Wm4 is positioned betweenthe local minimum value Ws3 and the local minimum value Ws4, and thisregion having a relatively large refractive index also becomes a coreportion 14. Here, the region between the local minimum value Ws1 and thelocal minimum value Ws2 functions as the core portion 141, and theregion between the local minimum value Ws3 and the local minimum valueWs4 functions as the core portion 142.

In FIG. 4, the region on the left side of the local minimum value Ws1,the region between the local minimum value Ws2 and the local minimumvalue Ws3, and the region on the right side of the local minimum valueWs4 represent regions adjacent to both side surfaces of the coreportions 14. These regions become the side cladding portions 15. Here,the region on the left side of the local minimum value Ws1 functions asthe side cladding portion 151 (first side cladding portion), the regionbetween the local minimum value Ws2 and the local minimum value Ws3functions as the side cladding portion 152 (second side claddingportion), and the region on the right side of the local minimum valueWs4 functions as the side cladding portion 153 (third side claddingportion).

In other words, in this example, the refractive index distribution W hasat least a region in which a second local maximum value included in alow refractive index region, a local minimum value, a first localmaximum value included in a high refractive index region, a localminimum value, and a separate second local maximum value different fromthe above second local maximum are arranged in sequence. This region isprovided repeatedly in accordance with the number of core portions. Whenthere are two core portions 14, as in the present embodiment, therefractive index distribution W has a region in which local maximumvalues and local minimum values are arranged alternately, and among thelocal maximum values, the first local maximum values and second localmaximum values are arranged alternately, such as a region in which asecond local maximum value, a local minimum value, a first local maximumvalue, a local minimum value, a second local maximum value, a localminimum value, a first local maximum value, a local minimum value, and asecond local maximum value are arranged in sequence.

Further, the plurality of local minimum values, the plurality of firstlocal maximum values, and the plurality of second local maximum valueseach preferably have substantially the same value. However, provided therelationships wherein the local minimum values are smaller than thefirst local maximum values and the second local maximum values, and thesecond local maximum values are smaller than the first local maximumvalues are maintained, slight differences in the values within eachplurality are acceptable. In this case, the difference is preferablykept within 10% of the average value of the plurality of local minimumvalues.

The four local minimum values Ws1, Ws2, Ws3 and Ws4 shown in FIG. 4 eachhas a value less than the value of the average refractive index WA inthe adjoining side cladding portions 15. As a result of this feature, aregion having an even smaller refractive index than the averagerefractive index of the side cladding portions 15 exists at the boundarybetween each core portion 14 and each side cladding portion 15.Consequently, a more steeply sloped refractive index gradient is formedin the vicinity of each of the local minimum values Ws1, Ws2, Ws3 andWs4. As a result, light leakage from each core portion 14 is bettersuppressed, and an optical waveguide 1 with less transmission loss canbe obtained.

Furthermore, within the refractive index distribution W illustrated inFIG. 4(b), the local maximum values Wm1, Wm3 and Wm5 (the second localmaximum values) are positioned within the side cladding portion 151, 152and 153. These local maximum values are preferably positioned away frompositions near the edges of the side cladding portions 151, 152 and 153(positions near the interfaces between the core portions 141 and 142 andthe side cladding portions). As a result of this feature, the localmaximum values Wm2 and Wm4 (the first local maximum values) within thecore portions 141 and 142, and the local maximum values Wm1, Wm3 and Wm5(the second local maximum values) within the side cladding portions 151,152 and 153 can be satisfactorily separated from each other.Consequently, the probability that transmitted light within the coreportions 141 and 142 will leak into the side cladding portion 151, 152and 153 can be satisfactorily reduced. As a result, transmission losswithin the core portions 141 and 142 can be reduced.

The expression “near the edges of the side cladding portions 151, 152and 153” describes regions that extend inward into each side claddingportion a distance of 5% of the width of each side cladding portion 151,152 and 153 from the aforementioned edge.

Furthermore, the local maximum values Wm1, Wm3 and Wm5 (the second localmaximum values) are preferably positioned in the central regions of thewidths of the side cladding portions 151, 152 and 153 respectively, andthe refractive index preferably decreases continuously from these localmaximum values Wm1, Wm3 and Wm5 (the second local maximum values) towardthe adjacent local minimum values Ws1, Ws2, Ws3 and Ws4. As a result ofthis feature, the separation distances between the local maximum valuesWm2 and Wm4 (the first local maximum values) within the core portions141 and 142, and the local maximum values Wm1, Wm3 and Wm5 (the secondlocal maximum values) within the side cladding portions 151, 152 and 153are maximized, and light can be reliably confined in the vicinity of thelocal maximum values Wm1, Wm3 and Wm5 (the second local maximum values).Consequently, leakage of transmitted light from the core portions 141and 142 can be more reliably suppressed.

Moreover, the local maximum values Wm1, Wm3 and Wm5 (the second localmaximum values) have a smaller refractive index than that of the localmaximum values Wm2 and Wm4 (the first local maximum values) positionedin the core portions 141 and 142. Accordingly, despite not having thesuperior light transmission properties of the core portions 141 and 142,because the refractive index is higher than that of the surroundingregions, some slight light transmission properties still exist. As aresult, the side cladding portion 151, 152 and 153 can confine anytransmitted light that leaks from the core portions 141 and 142, therebypreventing propagation of the light into other core portions. In otherwords, the existence of the local maximum values Wm1, Wm3 and Wm5 (thesecond local maximum values) can suppress crosstalk.

As mentioned above, each of the local minimum values Ws1, Ws2, Ws3 andWs4 has a refractive index that is less than the average refractiveindex WA of the adjoining side cladding portions 15. The differencepreferably falls within a prescribed range. Specifically, the differencebetween the local minimum values Ws1, Ws2, Ws3 and Ws4, and the averagerefractive index WA of the side cladding portions 15 is preferably about3 to 80%, more preferably about 5 to 50%, and still more preferablyabout 7 to 20%, of the difference between one of the local minimumvalues Ws1, Ws2, Ws3 and Ws4 or the average thereof, and one of thelocal maximum values Wm2 and Wm4 in the core portions 141 and 142 or theaverage thereof. As a result of this feature, the side cladding portions15 have the necessary light transmission properties for satisfactorilysuppressing crosstalk. Moreover, if the difference between the localminimum values Ws1, Ws2, Ws3 and Ws4, and the average refractive indexWA of the side cladding portions 15 is less than the lower limit of theabove range, then there is a possibility that the light transmissionproperties in the side cladding portions 15 may become too low, makingit difficult to satisfactorily suppress crosstalk. If the differenceexceeds the upper limit of the above range, there is a possibility thatthe light transmission properties in the side cladding portions 15 maybecome too great, which can have an adverse effect on the lighttransmission properties of the core portions 141 and 142.

Further, the difference between the local minimum values Ws1, Ws2, Ws3and Ws4, and the local maximum values Wm1, Wm3 and Wm5 (the second localmaximum values) is preferably about 6 to 90%, more preferably about 10to 70%, and still more preferably about 14 to 40% of the differencebetween the local minimum values Ws1, Ws2, Ws3 and Ws4, and the localmaximum values Wm2 and Wm4 (the first local maximum values). As aresult, an optimal balance is achieved between the height of therefractive index in the side cladding portions 15 and the height of therefractive index in the core portions 14, and therefore the opticalwaveguide 1 has particularly superior light transmission properties andcan more reliably suppress crosstalk.

The difference between the local minimum values Ws1, Ws2, Ws3 and Ws4,and the local maximum values Wm2 and Wm4 (the first local maximumvalues) within the core portions 141 and 142 is preferably as large aspossible, and is preferably from about 0.005 to 0.07, more preferablyfrom about 0.007 to 0.05, and still more preferably from about 0.01 to0.03. As a result, the refractive index difference is sufficient tosatisfactorily confine the light within the core portions 141 and 142.

The distance between the local maximum values Wm2 and Wm4 (the firstlocal maximum values) can be selected as required. For example, thelower limit of a preferred range is preferably at least 10 μm, morepreferably at least 20 μm, and still more preferably 30 μm or greater.Further, the upper limit is preferably not more than 800 μm, morepreferably not more than 500 μm, still more preferably not more than 400μm, and particularly preferably 300 μm or less. If necessary, thedistance may also be 200 μm or less, or even 100 μm or less.

FIG. 5 is a diagram illustrating the intensity distribution of theoutput light when light is incident upon only one core portion 141 ofthe optical waveguide 1 having the refractive index distributionillustrated in FIG. 4. This intensity distribution is the intensitydistribution of the output light at the other end of the opticalwaveguide when light is incident on one end (the input end) of the coreportion 141 among the two parallel core portions 141 and 142 formed inthe optical waveguide 1.

When light is incident upon the core portion 141, the intensity of theoutput light is greatest in the central portion of the core portion 141at the output end. The intensity of the output light decreases withincreasing separation from the central portion of the core portion 141.In the optical waveguide 1, an intensity distribution can be obtained inwhich the intensity of the output light has a local minimum value withinthe core portion 142 adjacent to the core portion 141. Because the localminimum value of the intensity distribution of the output lightcoincides with the core portion 142 in this manner, crosstalk in thecore portion 142 can be suppressed to an extremely low level. As aresult, even in multichannel and high-density configurations, theoptical waveguide 1 is capable of reliably preventing the occurrence ofcrosstalk.

In conventional optical waveguides, the intensity distribution of outputlight in a core portion adjacent to the core portion into which light isinput does not adopt a local minimum value, but rather adopts a localmaximum value. As a result, crosstalk problems tended to occur. Incontrast, the type of intensity distribution of the output light in theoptical waveguide of the present invention described above is extremelyuseful in suppressing crosstalk.

Although detailed reasons as to why this type of intensity distributionis obtained in the optical waveguide of the present invention remainunclear, one possible reason is described below. Specifically, thecharacteristic refractive index distribution W which has the localminimum values Ws1, Ws2, Ws3 and Ws4, and in which the refractive indexvaries continuously across the entire refractive index distribution W,shifts the local maximum value that appears at the core portion 142 inthe output light intensity distribution in a conventional opticalwaveguide to a local maximum value that appears at the side claddingportion 153 or the like adjacent to the core portion 142. In otherwords, this shift in the intensity distribution enables more reliablesuppression of crosstalk.

Even though the intensity distribution of the output light shifts towardthe side cladding portions 15, the light receiving elements and the likeare positioned in alignment with the positions of the core portions 14.As a result, there is almost no chance of crosstalk, and no degradationin the quality of the optical communication.

Further, the type of output light intensity distribution described abovecan be observed with high probability when at least two core portionsare formed in parallel in the optical waveguide of the presentinvention, but is not necessarily always observed. Depending on factorssuch as the NA (numerical aperture) of the incident light, thetransverse cross-sectional area of the core portion 141, and the pitchbetween the core portions 141 and 142, a clear local minimum value maynot be observable, or the position of the local minimum value may not beincluded in the core portion 142. However, even in these cases,crosstalk is still satisfactorily suppressed.

Furthermore, in the refractive index distribution W illustrated in FIG.4(b), when the average refractive index in the side cladding portions 15is denoted WA, the width of the portions in the vicinity of the localmaximum values Wm2 and Wm4 (the first local maximum values) where therefractive index is continuously equal to or greater than the averagerefractive index WA is denoted a [μm], and the width of the portions inthe vicinity of the local minimum values Ws1, Ws2, Ws3 and Ws4 where therefractive index is continuously less than the average refractive indexWA is denoted b [μm]. Then, b is preferably about 0.01a to 1.2a, morepreferably about 0.03a to 1a, and still more preferably about 0.1a to0.8a. As a result, the effective width of the local minimum values Ws1,Ws2, Ws3 and Ws4 is sufficient to achieve the actions and effectsdescribed above. In other words, if b is less than the above lowerlimit, then the effective width of the local minimum values Ws1, Ws2,Ws3 and Ws4 becomes too narrow, and there is a possibility that theaction of the core portions 141 and 142 in confining the light maydeteriorate. In contrast, if b exceeds the upper limit of the aboverange, then the effective width of the local minimum values Ws1, Ws2,Ws3 and Ws4 becomes too broad, resulting in corresponding limits on thewidth and pitch of the core portions 141 and 142, and there arepossibilities that the transmission efficiency may deteriorate, and thatmultichannel and high-density configurations may become less feasible.

The average refractive index WA in the side cladding portions 15 can beapproximated as the midpoint between the local maximum value Wm1 and thelocal minimum value Ws1.

(Cladding Layers)

The cladding layers 11 and 12 constitute the cladding portionspositioned respectively below and above the core layer 13.

The average thickness of the cladding layers 11 and 12 is preferablyabout 0.05 to 1.5 times, and more preferably about 0.1 to 1.25 times,the average thickness of the core layer 13 (the average height of eachcore portion 14). Specifically, although there are no particularlimitations on the average thickness of the cladding layers 11 and 12,each is typically about 1 to 200 μm, preferably about 3 to 100 μm, andmore preferably about 5 to 60 μm. This enables the optical waveguide 1to be prevented from becoming unnecessarily large (thick film), whileensuring favorable functionality as cladding portions.

Further, the constituent materials for the cladding layers 11 and 12 canbe selected as appropriate, and for example, the same materials as thosementioned above for the constituent material for the core layer 13 canbe used. A (meth)acrylic-based resin, epoxy-based resin orpolyolefin-based resin is particularly preferable.

When selecting the constituent material for the core layer 13 and theconstituent material for the cladding layers 11 and 12, the materialsare preferably selected with due consideration of the difference inrefractive index between the two materials. Specifically, in order toensure reliable confinement of the light in the core portions 14, thematerials are preferably selected so that the refractive index of theconstituent material for the core portions 14 is appropriately greater.This ensures that a satisfactory refractive index difference is obtainedin the thickness direction of the optical waveguide 1, and can suppressleakage of light from each core portion 14 into the cladding layers 11and 12.

From the viewpoint of suppressing light attenuation, it is alsoimportant to ensure good adhesion (affinity) between the constituentmaterial for the core layer 13 and the constituent material for thecladding layers 11 and 12.

On the other hand, there are no particular limitations on the shape ofthe refractive index distribution T in the thickness direction of theoptical waveguide 1, provided that the refractive index of the coreportion 14 is high and the refractive index of the cladding layers 11and 12 is low (for example, a step index type (SI type) distribution, ora so-called graded index type (GI type) distribution or W typedistribution in which the refractive index varies continuously may beused). However, the distribution preferably has a local maximum value atthe core portion 14, and local minimum values in the vicinity of theboundaries between the core portion 14 and the cladding layers 11 and12. The above expression that the “refractive index varies continuously”describes a state in which, in a similar manner to the refractive indexdistribution W described above, the curve of the refractive indexdistribution T is rounded at each portion of the distribution, with thiscurve being differentiable.

FIG. 6A(a) and FIG. 6B(c) are diagrams illustrating portions of thecross-sectional view along the line X-X shown in FIG. 2, and illustratecuts centered around a core portion that is sandwiched from above andbelow by the cladding layers. FIG. 6A(b) and FIG. 6B(d) are diagramsschematically illustrating examples of the refractive index distributionT along a centerline C2 which penetrates perpendicularly along thecenter in the width direction of the core portion. FIG. 6A(b) and FIG.6B(d) illustrate examples of the refractive index distribution T whenthe refractive index is shown along the horizontal axis, and theposition on the centerline C2 (the distance, wherein the center in thethickness direction of the core portion is deemed to be zero) is shownalong the vertical axis.

As described above, the optical waveguide 1 includes the cladding layer11, the core layer 13 and the cladding layer 12. Within a transversecross-section of the optical waveguide 1, the refractive indexdistribution T across the thickness direction of the core portion 14illustrated in FIG. 6A(b) has a local maximum value Tm positioned in thecentral portion, and local minimum values Ts1 and Ts2 positioned oneither side of the local maximum value Tm. The local minimum valuepositioned below the local maximum value Tm is denoted Ts1, and thelocal minimum value positioned above is denoted Ts2.

In the optical waveguide 1, as illustrated in FIG. 6A(b), the localmaximum value Tm is included between the local minimum value Ts1 and thelocal minimum value Ts2, and this region becomes the core portion 14.

On the other hand, the region below the local minimum value Ts1 becomesthe cladding layer 11, and the region above the local minimum value Ts2becomes the cladding layer 12.

In other words, the refractive index distribution T has at least aregion in which a local minimum value, a local maximum value and a localminimum value are arranged in sequence.

This region is provided repeatedly in accordance with the number oflaminated core layers. For example, when two core layers 13 are providedwith a cladding layer disposed therebetween, the refractive indexdistribution T contains local minimum values and local maximum valuesarrange alternately. In this case, the local maximum values preferablyinclude alternately arranged first local maximum values having arelatively large refractive index and second local maximum values havinga relatively small refractive index. In other words, the local maximumvalues are preferably arranged in a sequence composed of second localmaximum value, local minimum value, first local maximum value, localminimum value, second local maximum value, local minimum value, firstlocal maximum value . . . .

Further, the plurality of local minimum values, the plurality of firstlocal maximum values, and the plurality of second local maximum valueseach preferably have substantially the same value. However, provided therelationships wherein the local minimum values are smaller than thefirst local maximum values and the second local maximum values, and thesecond local maximum values are smaller than the first local maximumvalues are maintained, slight differences in the values within eachplurality are acceptable. In this case, the difference is preferablykept within 10% of the average value of the plurality of local minimumvalues.

The type of refractive index distribution T described above ismaintained as substantially the same distribution along the entirelongitudinal direction of the optical waveguide 1.

The local minimum value Ts1 is less than the average refractive index TAin the cladding layer 11. The local minimum value Ts2 is less than theaverage refractive index TA in the cladding layer 12. This ensures thata region having an even smaller refractive index than the averagerefractive index of each of the cladding layers 11 and 12 exists betweenthe core portion 14 and each of the cladding layers 11 and 12.Consequently, a more steeply sloped refractive index gradient is formedin the vicinity of each of the local minimum values Ts1 and Ts2. As aresult, light leakage from each core portion 14 into each of thecladding layers 11 and 12 is better suppressed, and an optical waveguide1 with less transmission loss can be obtained.

Further, in these refractive index distributions T, the refractive indexvaries continuously across the entire distribution. As a result, thelight confinement effect within the core portion 14 is enhanced comparedwith an optical waveguide having a step index type refractive indexdistribution. This enables further reductions in transmission loss to beachieved.

Moreover, in the refractive index distribution T, the distributionincludes the types of local minimum values Ts1 and Ts2 described above,and the refractive index varies continuously. Accordingly, because thespeed of the light is inversely proportional to the refractive index,the speed of the light increases with increasing distance from thecenter, and transmission time differences between optical paths areunlikely. As a result, the transmitted waveform is less likely todeteriorate. For example, even if the transmitted light contains a pulsesignal, rounding of the pulse signal (broadening of the pulse signal)can be suppressed. As a result, an optical waveguide 1 which can improvethe quality of optical communication can be obtained.

The expression that the refractive index varies continuously within therefractive index distribution T describes a state in which the curve ofthe refractive index distribution T is rounded at each portion of thedistribution, with this curve being differentiable.

Further, within the refractive index distribution T, as illustrated inFIG. 6A(b), the local maximum value Tm is positioned within the coreportion 14, and within this core portion 14, the local maximum value Tmis positioned in the central portion of the thickness. This increasesthe probability that, within the core portion 14, the transmitted lightwill be concentrated in the central portion of the thickness of the coreportion 14, resulting in a relative reduction in the probability thatlight will leak into the cladding layers 11 and 12. As a result,transmission loss in the core portions 141 and 142 can be furtherreduced.

The central portion of the thickness of the core portion 14 describesthe region from the midpoint between the local minimum value Ts1 and thelocal minimum value Ts2 to a distance 30% of the thickness of the coreportion 14 on both sides of the midpoint.

Furthermore, the position of the local maximum value Tm need notnecessarily be in the central portion, and may be positioned in anylocation away from positions near the edges of the core portion 14 (nearthe interfaces with each of the cladding layers 11 and 12). This enablestransmission loss within the core portion 14 to be suppressed to somedegree.

The expression “near the edges of the core portion 14” describes aregion that extends inward into the core portion a distance of 5% of thethickness of the core portion 14 from the aforementioned edge.

On the other hand, in the refractive index distribution T, within eachof the cladding layers 11 and 12, the refractive index varies in such amanner that it is highest at a position that is not near the interfacewith the core portion 14, and is lowest near the interface with the coreportion 14. As a result, the local maximum value Tm within the coreportion 14 and the regions of high refractive index within each of thecladding layers 11 and 12 are satisfactorily separated. Accordingly, theprobability that transmitted light in the core portion 14 will leak intothe cladding layers 11 and 12 can be satisfactorily reduced. As aresult, transmission loss within the core portion 14 can be reduced.

In each of the cladding layers 11 and 12, the expression “near theinterface with the core portion 14” describes a region that extendsinward into the cladding layer 11 or 12 a distance of 5% of thethickness of the cladding layer 11 or 12 from the interface.

The average refractive index TA in each of the cladding layers 11 and 12can be approximated as the midpoint between the local minimum value Ts1or Ts2 respectively and the maximum value in each of the cladding layers11 and 12.

Further, as mentioned above, the local minimum values Ts1 and Ts2 areless than the average refractive index TA in each of the cladding layers11 and 12. It is desirable that the difference between the two valuesfalls within a prescribed range. Specifically, the difference betweenthe local minimum values Ts1 and Ts2, and the average refractive indexTA of the cladding layers 11 and 12 is preferably about 3 to 80%, morepreferably about 5 to 50%, and still more preferably about 7 to 30%, ofthe difference between the local minimum values Ts1 and Ts2 and thelocal maximum value Tm in the core portion 14. As a result, the claddinglayers 11 and 12 have the necessary light transmission properties forsatisfactorily suppressing crosstalk. Moreover, if the differencebetween the local minimum values Ts1 and Ts2 and the average refractiveindex TA of the cladding layers 11 and 12 is less than the above lowerlimit, then there is a possibility that the light transmissionproperties in each of the cladding layers 11 and 12 may become too low,making it difficult to satisfactorily suppress crosstalk. On the otherhand, if the difference exceeds the above upper limit, there is apossibility that the light transmission properties in each of thecladding layers 11 and 12 may become too great, which can have anadverse effect on the light transmission properties of the core portion14.

Furthermore, the refractive index difference between the local minimumvalues Ts1 and Ts2 and the local maximum value Tm in the core portion 14is preferably as large as possible. The difference can be selected asrequired, but is preferably from about 0.005 to 0.07, more preferablyfrom about 0.007 to 0.05, and still more preferably from about 0.01 to0.05. This ensures that the aforementioned refractive index differenceis sufficient to achieve the necessary light confinement within the coreportion 14.

As mentioned above, and as illustrated in FIG. 6B(d), the refractiveindex distribution T may also be a so-called graded index typedistribution. The refractive index distribution T illustrated in FIG.6B(d) has a local maximum value Tm in the core portion 14, and has aconstant refractive index that is less than the local maximum value Tmin the cladding layers 11 and 12.

(Intersection Portions)

FIG. 7A is a top view illustrating the optical waveguide 1 shown in FIG.2 (with the upper layers 3 and 12 omitted). FIG. 7B is a diagramillustrating the refractive index distribution in the vicinity of anintersection portion.

The intersection portions 147 between the core portions 141 (first coreportion) and 142 (third core portion), and the core portion 145 (secondcore portion) may have a refractive index distribution such as thatillustrated in FIG. 7B, with a local maximum value in the centralportion and two tailing portions in which the refractive index graduallydecreases from the local maximum value toward the periphery, oralternatively, the refractive index distribution of the intersectionportion 147 may be uniform. In the former case, signal light is moreeasily concentrated in the central portion of the intersection portion147, making interference less likely. In the latter case, the signallight can proceed linearly through the intersection portion 147, therebysuppressing transmission in unintended directions.

The refractive index of each intersection portion 147 is preferablyhigher than that of the surrounding region. Based on this refractiveindex difference, signal light entering the intersection portion 147 isunlikely to enter the core portion which intersects the core portionthrough which the signal light has been transmitted. As a result, in theoptical waveguide 1, core portions can intersect within the same planewithout signal light interference at the intersection portions 147.

Furthermore, by intersecting a plurality of core portions within thesame plane, signal intersection can be achieved without resorting tothree dimensional intersections. Accordingly, by using this type ofoptical waveguide 1, miniaturization, reduced thickness and higherdensity can be achieved for devices in which the optical waveguide 1 isinstalled.

The maximum refractive index of the intersection portion 147 ispreferably about 0.001 to 0.05 larger, and more preferably about 0.002to 0.03 larger, than the local maximum value Wm of the refractive indexdistribution W.

The constituent material of the intersection portions 147 may bedifferent from other regions which constitute the core layer 13. In sucha case, following formation of the core layer 13, a portion of the layercan be removed and filled with another material, thereby formingintersection portions 147 having a uniform refractive indexdistribution. Examples of this other material include the polymersdescribed below, which may be selected as appropriate in accordance withthe magnitude relationship with the refractive index of the coreportions 14.

Furthermore, a “uniform refractive index distribution” means that thefluctuation in the refractive index in the intersection portion 147 isnot more than 5% of the average refractive index in the intersectionportion 147.

If the optical axis of the core portion 141 (the first core portion) isdenoted A1, the optical axis of the core portion 142 (the third coreportion) is denoted A2, and the optical axis of the core portion 145(the second core portion) is denoted A5, then the angle of intersectionof the optical axis A1 and the optical axis A5, and the angle ofintersection of the optical axis A2 and the optical axis A5, are eachpreferably from 10 to 90°, and more preferably from 20 to 90°. Providedthe angle of intersection satisfies this range, interference can besatisfactorily suppressed.

Further, in the core portions 141, 142 and 145 of the refractive indexdistribution W described above, because signal light is transmittedconcentrated near the local maximum value, interference at theintersection portions 147 is unlikely. However, by appropriate selectionof the above conditions, attenuation in the intersection portions 147can also be suppressed. Specifically, when the angle of intersection ofthe optical axis A1 and the optical axis A5 is 90°, the opticalwaveguide 1 of the present invention exhibits transmission loss of 0.02dB or less in the intersection portion 147. With this type of opticalwaveguide 1, even if a plurality of intersection portions 147 areformed, transmission loss can still be suppressed to a low level,meaning complex optical wiring can be constructed.

FIG. 8A and FIG. 8B are partial enlargements illustrating other examplesof the vicinity of the intersection portion.

The optical waveguides illustrated in FIG. 8A and FIG. 8B each have aconfiguration in which, in the vicinity of the intersection portion 147,the widths of the core portion 141 and the core portion 145 graduallyincrease as they approach the intersection portion 147. Of theseconfigurations, in the optical waveguide illustrated in FIG. 8A, thewidths of the core portion 141 and the core portion 145 increasegradually in a linear manner. On the other hand, in the opticalwaveguide illustrated in FIG. 8B, the widths of the core portion 141 andthe core portion 145 increase gradually along a curve. With these typesof configurations, interference in the intersection portion 147 isparticularly well suppressed, and the transmission efficiency in theintersection portion 147 can be improved.

Further, FIG. 7A illustrates an example in which the core portion 141and the core portion 145, and the core portion 142 and the core portion145 intersect at different intersection portions 147, but all three ofthe core portion 141, the core portion 142 and the core portion 145 mayalso intersect at the same intersection portion. Alternatively, an evengreater number of core portions may intersect at the intersectionportion. FIG. 8C and FIG. 8D are diagrams illustrating the latter casein which three or more core portions intersect.

In the intersection portion 148 illustrated in FIG. 8C, the optical axisA1 of the core portion 141, the optical axis A2 of the core portion 142and the optical axis A5 of the core portion 145 intersect at a singlepoint such that all of the internal angles formed are 60°.

Moreover, in the intersection portion 148 illustrated in FIG. 8D, fourcore portions composed of the core portion 141, the core portion 142,the core portion 145 and a core portion 146 all intersect at the sameintersection portion. In the intersection portion 148 illustrated inFIG. 8D, the optical axis A1 of the core portion 141, the optical axisA2 of the core portion 142, the optical axis A5 of the core portion 145and the optical axis A6 of the core portion 146 intersect at a singlepoint such that all of the internal angles formed are 45°.

By including these types of intersection portions 148, the opticalwaveguide 1 can be constructed with a higher density and with morecomplex optical wiring. The number of core portions intersecting at theintersection portion 148 can be selected as required, and may even be 5or more core portions. Further, in the intersection portion 148, thenumber of intersecting core portions is preferably adjustedappropriately so that the internal angles formed are within a range from10 to 80°, more preferably from 20 to 70°, and still more preferablyfrom 30 to 60°. Moreover, the plurality of internal angles formed may beeither equal or different.

(Mirror)

A mirror may be provided in the optical waveguide 1 if required.

A mirror may be formed partway along the core portions 14 of the opticalwaveguide 1.

(Support Film)

If required, a support film 2 such as that illustrated in FIG. 2 may belaminated to the lower surface of the optical waveguide 1.

The support film 2 may be selected as appropriate, and supports,protects and reinforces the lower surface of the optical waveguide 1.This can enhance the reliability and improve the mechanical propertiesof the optical waveguide 1.

<<Second Embodiment>>

Next is a description of a second embodiment of the optical waveguide ofthe present invention.

FIG. 9 is a perspective view (with a portion cut away, and illustratedas partially transparent) illustrating the second embodiment of theoptical waveguide of the present invention. In the followingdescription, the upper side in FIG. 9 is referred to using the term“upper” and the lower side is referred to using the term “lower”.

The second embodiment of the optical waveguide is described below, withthe description focusing on the points of difference from the firstembodiment. Descriptions are omitted for items that are the same as thefirst embodiment. In FIG. 9, those structural components that are thesame as the first embodiment are labeled with the same numerals as inthe preceding description, and detailed descriptions of those componentsare omitted.

With the exception of having two laminated core layers 13 with acladding layer disposed therebetween, the second embodiment is the sameas the first embodiment. In other words, the optical waveguide 1illustrated in FIG. 9 is composed of five layers, specifically acladding layer 11, a core layer 13, a cladding layer 121, a core layer13 and a cladding layer 122, laminated in that sequence from the lowerside.

Of these layers, the two core layers 13 are the same as the core layer13 of the first embodiment, and are each formed from two core portions14 (a first core portion and a third core portion) arranged in parallelin the width direction, a single core portion 14 (a second core portion)which intersects these two core portions 14, and side cladding portions15 which adjoin these core portions 14.

More specifically, of the two core layers 13 illustrated in FIG. 9, thelower core layer 131 is formed from two parallel core portions 141 (afirst core portion) and 142 (a third core portion), a single coreportion 145 (a second core portion) which intersects each of these coreportions 141 and 142, and side cladding portions 151, 152 and 153 whichadjoin these core portions 141, 142 and 145.

On the other hand, the upper core layer 132 is formed from two parallelcore portions 143 (a first core portion) and 144 (a third core portion),a single core portion 145 (the second core portion) which intersectseach of these core portions 143 and 144, and side cladding portions 154,155 and 156 which adjoin these core portions 143, 144 and 145.

Further, as illustrated in FIG. 9, in each of the core layers 131 and132, the core portions 14 (141, 142, 143, 144 and 145) are provided inprescribed positions and combinations so as to be superimposed whenviewed in plan view.

In the optical waveguide 1 illustrated in FIG. 9, a refractive indexdistribution T is formed in which the refractive index varies in thethickness direction. This refractive index distribution T has regions ofrelatively high refractive index and regions of relatively lowrefractive index, and therefore incident light can be transmitted withthe light confined to the regions of high refractive index.

One example of this refractive index distribution T is described below.

FIG. 10(a) is a diagram illustrating a portion of the cross-sectionalong the line Y-Y shown in FIG. 9, and illustrates a cut through thetwo core portions sandwiched between the cladding layers. FIG. 10(b)schematically illustrates one example of the refractive indexdistribution T along a centerline C2′ which penetrates along the centerof the width of the core portions in this cross-sectional view along theline Y-Y. FIG. 10(b) schematically illustrates an example of therefractive index distribution in the thickness direction when therefractive index is shown along the horizontal axis, and the position(distance) along the thickness direction of the core portions in thetransverse cross-section is shown along the vertical axis. A positionfarther right on the horizontal axis indicates a larger refractiveindex.

The optical waveguide 1 has a refractive index distribution T such asthat illustrated in FIG. 10(b), having four local minimum values Ts1,Ts2, Ts3 and Ts4, and five local maximum values Tm1, Tm2, Tm3, Tm4 andTm5. Further, the five local maximum values include local maximum valuesof relatively large refractive index (first local maximum values) Tm2and Tm4, and local maximum values of relatively small refractive index(second local maximum values) Tm1, Tm3 and Tm5.

Among these, the local maximum values Tm2 and Tm4 of relatively largerefractive index exist between the local minimum value Ts1 and the localminimum value Ts2, and the local minimum value Ts3 and the local minimumvalue Ts4 respectively. The remaining local maximum values Tm1, Tm3 andTm5 are local maximum values of relatively small refractive index.

The local minimum value Ts1 is positioned on the boundary line betweenthe cladding layer 11 and the core portion 141, the local minimum valueTs2 is positioned on the boundary line between the core portion 141 andthe cladding layer 121, the local minimum value Ts3 is positioned on theboundary line between the cladding layer 121 and the core portion 143,and the local minimum value Ts4 is positioned on the boundary linebetween the core portion 143 and the cladding layer 122.

Further, the local maximum values Tm2 and Tm4 (the first local maximumvalues) are preferably positioned in the central portions of the coreportions 141 and 143 respectively. On the other hand, the local maximumvalues Tm1, Tm3 and Tm5 (the second local maximum values) are preferablypositioned in the central portions of the cladding layers 11, 121 and122 respectively.

In other words, the refractive index distribution T has at least aregion in which a second local maximum value, a local minimum value, afirst local maximum value, a local minimum value, and a second localmaximum value are arranged in sequence. This region can be providedrepeatedly in accordance with the number of core layers. When there aretwo laminated core layers 13, as in the present embodiment, therefractive index distribution T has a region in which local maximumvalues and local minimum values are arranged alternately, and among thelocal maximum values, the first local maximum values and the secondlocal maximum values are arranged alternately, such as a region in whicha second local maximum value, a local minimum value, a first localmaximum value, a local minimum value, a second local maximum value, alocal minimum value, a first local maximum value, a local minimum value,and a second local maximum value are arranged in sequence.

The four local minimum values Ts1, Ts2, Ts3 and Ts4 are less than theaverage refractive index TA in the adjoining cladding layers 11, 121 and122. This ensures that a region having an even smaller refractive indexthan the average refractive index TA of each of the cladding layers 11,121 and 122 exists between each of the core portions 14 and each of thecladding layers 11, 121 and 122. Consequently, a more steeply slopedrefractive index gradient is formed in the vicinity of each of the localminimum values Ts1, Ts2, Ts3 and Ts4. As a result, light leakage fromeach core portion 14 is better suppressed, and an optical waveguide 1with minimal transmission loss and good suppression of crosstalk in thethickness direction can be obtained.

Further, in the refractive index distribution T, the refractive indexvaries continuously across the entire distribution. As a result, thelight confinement effect within the core portions 14 is enhancedcompared with an optical waveguide having a step index type refractiveindex distribution. This enables a further reduction in transmissionloss and a greater suppression of crosstalk to be achieved.

On the other hand, within the refractive index distribution T, asillustrated in FIG. 10(b), the local maximum values Tm1, Tm3 and Tm5(the second local maximum values) are positioned within the claddinglayers 11, 121 and 122 respectively. These local maximum values arepreferably positioned away from positions near the edges of the claddinglayers 11, 121 and 122 (positions near the interfaces with the coreportions 141 and 143). As a result, the local maximum values Tm2 and Tm4(the first local maximum values) within the core portions 141 and 143,and the local maximum values Tm1, Tm3 and Tm5 (the second local maximumvalues) within the cladding layers 11, 121 and 122 can be satisfactorilyseparated from each other. Consequently, the probability thattransmitted light within the core portions 141 and 143 will leak intothe cladding layers 11, 121 and 122 can be satisfactorily reduced. As aresult, transmission loss within the core portions 141 and 143 can bereduced, and crosstalk can be better suppressed.

The expression “near the edges of the cladding layers 11, 121 and 122”describes regions that extend inward into each cladding layer a distanceof 5% of the thickness of each cladding layer 11, 121 and 122 from theaforementioned edge.

Furthermore, the local maximum values Tm1, Tm3 and Tm5 (the second localmaximum values) are preferably positioned in the central regions of thethickness of the cladding layers 11, 121 and 122 respectively, and therefractive index preferably decreases continuously from these localmaximum values Tm1, Tm3 and Tm5 toward the adjacent local minimum valuesTs1, Ts2, Ts3 and Ts4. As a result, the separation distances between thelocal maximum values Tm2 and Tm4 (the first local maximum values) withinthe core portions 141 and 143, and the local maximum values Tm1, Tm3 andTm5 (the second local maximum values) within the cladding layers 11, 121and 122 is maximized, and light can be reliably confined in the vicinityof the local maximum values Tm1, Tm3 and Tm5. Consequently, leakage oftransmitted light from the core portions 141 and 143 can be morereliably suppressed.

The central portion of the thickness of the cladding layer 121 describesthe region from the midpoint between the local minimum value Ts2 and thelocal minimum value Ts3 to a distance 30% of the thickness of thecladding layer 121 on both sides of the midpoint.

Moreover, the local maximum values Tm1, Tm3 and Tm5 are local maximumvalues having a lower refractive index than that of the local maximumvalues Tm2 and Tm4 (the first local maximum values) positioned in thecore portions 141 and 143. Accordingly, despite not having the superiorlight transmission properties of the core portions 141 and 143, becausethe refractive index is higher than that of the surrounding regions,some slight light transmission properties still exist. As a result, thecladding layers 11, 121 and 122 can confine any transmitted light thatleaks from the core portions 141 and 143, thereby preventing propagationof the light into other core portions. In other words, the existence ofthe local maximum values Tm1, Tm3 and Tm5 can more reliably suppresscrosstalk.

As mentioned above, each of the local minimum values Ts1, Ts2, Ts3 andTs4 has a refractive index that is less than the average refractiveindex TA of each of the cladding layers 11, 121 and 122, and therefractive index difference preferably falls within a prescribed range.Specifically, the difference between the local minimum values Ts1, Ts2,Ts3 and Ts4, and the average refractive index TA of each of the claddinglayers 11, 121 and 122 is preferably about 3 to 80%, more preferablyabout 5 to 50%, and still more preferably about 7 to 30%, of thedifference between the local minimum values Ts1, Ts2, Ts3 and Ts4, andthe local maximum values Tm2 and Tm4 (the first local maximum values) inthe core portions 141 and 143. As a result, each of the cladding layers11, 121 and 122 has the necessary light transmission properties forsatisfactorily suppressing crosstalk. If the difference between thelocal minimum values Ts1, Ts2, Ts3 and Ts4, and the average refractiveindex TA of each of the cladding layers 11, 121 and 122 is less than thelower limit of the above range, then there is a possibility that thelight transmission properties in each of the cladding layers 11, 121 and122 may become too low, making it difficult to satisfactorily suppresscrosstalk. On the other hand, if the difference exceeds the upper limitof the above range, there is a possibility that the light transmissionproperties in each of the cladding layers 11, 121 and 122 may become toogreat, which can have an adverse effect on the light transmissionproperties of the core portions 141 and 143.

Further, the difference between the local minimum values Ts1, Ts2, Ts3and Ts4, and the local maximum values Tm1, Tm3 and Tm5 (the second localmaximum values) is preferably about 6 to 90%, more preferably about 10to 70%, and still more preferably about 14 to 40% of the differencebetween the local minimum values Ts1, Ts2, Ts3 and Ts4, and the localmaximum values Tm2 and Tm4 (the first local maximum values). As aresult, an optimal balance is achieved between the height of therefractive index in the cladding layers and the height of the refractiveindex in the core portions, and therefore the optical waveguide 1 hasparticularly superior light transmission properties and can morereliably suppress crosstalk.

Furthermore, in the refractive index distribution T illustrated in FIG.10(b), when the average refractive index in each of the cladding layers11, 121 and 122 is denoted TA, the width of the portions in the vicinityof the local maximum values Tm2 and Tm4 (the first local maximum values)where the refractive index is continuously equal to or greater than theaverage refractive index TA is denoted a [μm], and the width of theportions in the vicinity of the local minimum values Ts1, Ts2, Ts3 andTs4 where the refractive index is continuously less than the averagerefractive index TA is denoted b [μm] (wherein a and b are set in thesame manner as described for FIG. 4). Then, b is preferably about 0.01ato 1.2a, more preferably about 0.03a to 1a, and still more preferablyabout 0.1a to 0.8a. As a result, the effective width of the localminimum values Ts1, Ts2, Ts3 and Ts4 is sufficient to satisfactorilyachieve the actions and effects described above. In other words, if b isless than the above lower limit, then the effective width of the localminimum values Ts1, Ts2, Ts3 and Ts4 becomes too narrow, and there is apossibility that the action of the core portions 141 and 143 inconfining the light may deteriorate. In contrast, if b exceeds the upperlimit of the above range, then the effective width of the local minimumvalues Ts1, Ts2, Ts3 and Ts4 becomes too broad, resulting incorresponding limits on the thickness and pitch of the core portions 141and 143, and there are possibilities that the transmission efficiencymay deteriorate, and that multichannel and high-density configurationsmay become less feasible.

The average refractive index TA in the cladding layer 11 can beapproximated as the midpoint between the local maximum value Tm1 and thelocal minimum value Ts1.

Further, in the present embodiment, crosstalk between the core portions141 and 143 that are arranged in the thickness direction of the opticalwaveguide 1 can also be suppressed.

Specifically, among the plurality of core portions 141, 142, 143 and 144of the optical waveguide 1 illustrated in FIG. 9, if light is input intoone end of a single selected core portion, and the intensitydistribution P2 of the output light is acquired at the other end of thecore portions, then the intensity distribution displays a characteristicdistribution which is ideal for suppressing crosstalk.

FIG. 11 is a diagram illustrating the intensity distribution P2 of theoutput light in a portion of the output end surface when light is inputinto only a single core portion 141 of the optical waveguide 1illustrated in FIG. 9. Specifically, the diagram illustrates one exampleof the intensity distribution, wherein the intensity of the output lightis shown along the horizontal axis, and the position on the output endsurface (the distance in the thickness direction) is shown along thevertical axis.

When light is incident on the core portion 141 (CH1), the intensity ofthe output light is greatest in the central portion of the output end ofthe core portion 141. The intensity of the output light decreases withincreasing separation from the central portion of the core portion 141,and adopts a local minimum value in the core portion 143 (CH2) adjacentto the core portion 141 in the thickness direction. In other words, theintensity distribution P2 of the output light in this case adopts alocal maximum value Pml in the central portion of the output end of thecore portion 141 (CH1), and adopts a local minimum value Psi in the coreportion 143 (CH2). In an optical waveguide 1 in which the output lighthas this type of intensity distribution, although leakage of the lighttransmitted through the core portion 141 cannot be completely prevented,concentration of the leaked light in the core portion 143 is suppressed.Accordingly, “crosstalk” where the leaked light causes interference inthe core portion 143 can be reliably suppressed. As a result, theoptical waveguide 1 can reliably prevent crosstalk, even forconfigurations having multiple channels and increased density not onlyin the width direction, but also in the thickness direction.

In the present embodiment, in a similar manner to that described above,the refractive index distribution T may also be a so-called step indextype distribution or a graded index type distribution.

(Connectors)

The connectors 101 are provided at the ends of the optical waveguide 1,and can optically connect the core groups 140 with other opticalcomponents. These connectors 101 may conform to any of various connectorstandards. Examples of connectors which conform to connector standardsinclude Mini MT connectors, MT connectors prescribed in JIS C 5981, 16MTconnectors, two dimensional array MT connectors, MPO connectors, and MPXconnectors.

When the connectors 101 are installed on the optical waveguide 1, theends of the core groups 140 are exposed at the end surfaces of theconnectors 101. By connecting other connectors to these connectors 101,optical components such as other optical waveguides or optical fiberscan be connected optically to the core groups 140. Examples of opticalcomponents that may be connected include not only optical waveguides andoptical fibers, but also wavelength conversion elements, filters,diffraction gratings, polarizers, prisms and lenses.

Further, examples of the constituent material for the connectors 101include resin materials, metal materials and ceramic materials.

Furthermore, there are no particular limitations on the installationstructure of the connectors 101, and structures in which the connectors101 protrude from the end surfaces of the optical waveguide 1 may alsobe used. In such cases, notches or the like need not be provided in theoptical waveguide 1.

The pattern of the core groups 140 in the optical waveguide 1 is notlimited to that illustrated in FIG. 1, and any pattern may be used.

FIG. 12 is a plan view (shown through the cladding layer) illustratingan example of another configuration for the first embodiment of theoptical wiring component of the present invention.

The optical waveguide 1 in FIG. 12 is the same as the optical waveguide1 illustrated in FIG. 1, with the exception that within the core groups140, each of which is an assembly of four core portions 14 initiallyaligned in parallel, each core portion 14 is configured so as to splitoff into a different direction partway along the portion, with each coreportion 14 being connected to a different connector 101. In an opticalwiring component 10 containing an optical waveguide 1 with this type ofpattern, the same effects as those described above can be obtained. Inother words, complex and high-density signal paths such as those whichmutually intersect within the same plane can be constructed without anaccompanying increase in the thickness of the waveguide, and thereforean optical wiring component 10 can be obtained which can be easily bent,and which enables wiring operations to be performed with relative ease,even in confined wiring spaces.

<Method of Producing Optical Waveguide>

Next is a description of one example of a method of producing theoptical waveguide of the present invention.

The optical waveguide 1 may be produced by sequential film formationusing a composition for forming the cladding layer 11, a composition forforming the core layer 13, and a composition for forming the claddinglayer 12, but can also be produced by simultaneous extrusion molding ofa plurality of compositions into a plurality of layers, such as thesimultaneous extrusion molding of three compositions into three layers.The latter method is described below.

FIG. 13 to FIG. 15 are diagrams for describing a method of producing theoptical waveguide illustrated in FIG. 2. In the following description,the upper side in FIG. 13 to FIG. 15 is referred to using the term“upper” and the lower side is referred to using the term “lower”.

The method of producing the optical waveguide 1 may include, forexample, the steps described below.

[1] First, a desired number of layers of two types of opticalwaveguide-forming compositions 901 and 902 (a first composition and asecond composition) are layered alternately on a support substrate 951,preferably by extrusion molding, thereby obtaining a layer 910.

[2] Next, a portion of the layer 910 is irradiated with active radiationto generate a refractive index difference, thus obtaining the opticalwaveguide 1.

Each of these steps is described below in sequence.

Step [1]

First, the optical waveguide-forming compositions 901 and 902 areprepared.

The optical waveguide-forming compositions 901 and 902 each contain apolymer 915 and an additive 920 (which includes at least a monomer inthe present embodiment). However, the compositions are different fromeach other.

Among the two compositions, the optical waveguide-forming composition901 is a material used mainly for forming the core layer 13.Specifically, the optical waveguide-forming composition 901 is amaterial in which irradiation with active radiation causes an activereaction of at least the monomer in the polymer 915, resulting in anaccompanying change in the refractive index distribution. In otherwords, the optical waveguide-forming composition 901 is a material inwhich a change in the refractive index distribution is generated due tovariation in the content ratio between the polymer 915 and the monomer,thereby enabling the formation of the core portions 14 and the sidecladding portions 15 within the core layer 13.

On the other hand, the optical waveguide-forming composition 902 is amaterial used mainly for forming the cladding layers 11 and 12, and iscomposed of a material having a lower refractive index than that of thematerial of the optical waveguide-forming composition 901.

The difference in refractive index between the optical waveguide-formingcomposition 901 and the optical waveguide-forming composition 902 can beadjusted as appropriate by setting the composition of the polymers 915and the composition of the monomers contained in each composition, andsetting the content ratio between the polymer 915 and the monomer.

For example, when the refractive index of the monomer is lower than thatof the polymer 915, the monomer content within the compositions ispreferably higher for the optical waveguide-forming composition 902 thanfor the optical waveguide-forming composition 901. On the other hand,when the refractive index of the monomer is higher than that of thepolymer 915, the monomer content within the compositions is preferablyhigher for the optical waveguide-forming composition 901 than for theoptical waveguide-forming composition 902. In other words, the polymers915 and the additives 920 (including the monomer) in the opticalwaveguide-forming compositions 901 and 902 are selected appropriately inaccordance with the refractive indices of each of the polymers 915 andmonomers.

Further, the compositions of the optical waveguide-forming composition901 and the optical waveguide-forming composition 902 are preferably setsuch that the monomer content is substantially equal within eachcomposition. This enables the difference in monomer content between theoptical waveguide-forming composition 901 and the opticalwaveguide-forming composition 902 to be reduced. As a result, diffusionand migration of the monomers due to the difference in monomer contentcan be suppressed. As mentioned above, diffusion and migration of themonomers is sometimes useful in forming a refractive index difference,but if the difference in the content values is large, then migration ofthe monomers in an undesirable direction is sometimes unavoidable. In amultilayer (multicolor) extrusion molding method described below, arefractive index distribution in the thickness direction of the layer910 can be freely formed. Accordingly, there is no problem withsuppressing diffusion and migration of the monomers at least within thethickness direction, and it is actually preferable that unintendeddiffusion and migration of the monomers in the thickness direction issuppressed. By suppressing such unintended diffusion and migration ofthe monomers, an optical waveguide 1 having a refractive indexdistribution T of the targeted shape can be produced more reliably.

When the monomer content values are substantially equal, it ispreferable that the polymer 915 or the monomer conditions is differentbetween the optical waveguide-forming composition 901 and the opticalwaveguide-forming composition 902. Specifically, the compositions of thepolymers 915 that are used may be different between the opticalwaveguide-forming composition 901 and the optical waveguide-formingcomposition 902, or even if the compositions of the polymers 915 are thesame, the polymer molecular weights or the polymerization degrees may bedifferent. Further, the compositions of the monomers, and therefore therefractive indices thereof, may be different. By so doing, a refractiveindex difference can be formed between the optical waveguide-formingcomposition 901 and the optical waveguide-forming composition 902, eventhough the monomer content values are substantially equal, therebysuppressing diffusion and migration of the monomers.

Next is a description of the method used for molding the opticalwaveguide-forming compositions 901 and 902 in layers on top of thesupport substrate 951 using a multilayer extrusion molding method.

In the multilayer extrusion molding method, for example, by extrudingthree layers of the optical waveguide-forming composition 901, and alsoextruding a layer of the optical waveguide-forming composition 902between each pair of these layers of the optical waveguide-formingcomposition 901, a multilayer compact 914 composed of five layer isformed in a single operation.

Specifically, in the multilayer compact 914, the opticalwaveguide-forming composition 901, the optical waveguide-formingcomposition 902, the optical waveguide-forming composition 901, theoptical waveguide-forming composition 902, and the opticalwaveguide-forming composition 901 are extruded simultaneously in thissequence from the lower side. Consequently, at the boundaries betweenthe compositions, slight mixing of the optical waveguide-formingcomposition 901 and the optical waveguide-forming composition 902occurs. Accordingly, in the vicinity of the boundaries between thecompositions, a portion of the optical waveguide-forming composition 901and a portion of the optical waveguide-forming composition 902 are mixedtogether, forming a region in which the mixing ratio varies continuouslyalong the thickness direction.

As a result, the multilayer compact 914 has a structure in which a firstmolded layer 914 a formed mainly from the optical waveguide-formingcomposition 901, a second molded layer 914 b formed from a mixture ofthe optical waveguide-forming composition 901 and the opticalwaveguide-forming composition 902, a third molded layer 914 c formedmainly from the optical waveguide-forming composition 902, a fourthmolded layer 914 d formed from a mixture of the opticalwaveguide-forming composition 901 and the optical waveguide-formingcomposition 902, a fifth molded layer 914 e formed mainly from theoptical waveguide-forming composition 901, a sixth molded layer 914 fformed from a mixture of the optical waveguide-forming composition 901and the optical waveguide-forming composition 902, a seventh moldedlayer 914 g formed mainly from the optical waveguide-forming composition902, an eighth molded layer 914 h formed from a mixture of the opticalwaveguide-forming composition 901 and the optical waveguide-formingcomposition 902, and a ninth molded layer 914 i formed mainly from theoptical waveguide-forming composition 901 are laminated together in thissequence from the lower side, as shown in the layers of FIG. 13(a).

Then, the solvent within the obtained multilayer compact 914 isevaporated (desolvation) to obtain the layer 910. (see FIG. 13(b)).

The obtained layer 910 is formed as a laminate containing, in sequencefrom the lower side in FIG. 13(b), the cladding layer 11 which iscomposed of the layers below the central portion of the third moldedlayer 914 c, the core layer 13 which is composed of the layers above thecentral region of the third molded layer 914 c and below the centralportion of the seventh molded layer 914 g, and the cladding layer 12which is composed of the layers above the central portion of the seventhmolded layer 914 g. The core layer 13 has a higher refractive index thanthat of the cladding layers 11 and 12.

In the obtained layer 910, the polymer (matrix) 915 exists in anessentially uniform and random manner in the width direction. Further,the additive 920 is dispersed essentially uniformly and randomly withinthe polymer 915. As a result, the additive 920 is dispersed essentiallyuniformly and randomly within the layer 910.

There are no particular limitations on the average thickness of thelayer 910, and the thickness may be set appropriately in accordance withthe thickness of the optical waveguide 1 that is to be formed. However,the average thickness of the layer 910 is preferably about 10 to 500 μm,and more preferably about 20 to 300 μm.

The support substrate 951 can be selected as appropriate, and forexample, a silicon substrate, silicon dioxide substrate, glasssubstrate, or a polyethylene terephthalate (PET) film or the like can beused.

However, the multilayer compact 914 used in obtaining the above type oflayer 910 is produced using a die coater (multilayer extrusion moldingapparatus) or the like, which can be selected as appropriate.

If it is desirable to adjust the distribution in the thicknessdirection, then, for example, by forming the first molded layer 914 aand the ninth molded layer 914 i significantly thinner than the othermolded layers such as the fifth molded layer 914 e, the refractive indexof the lowermost layer portion and the lowermost layer portion can beprevented from becoming higher than the refractive index of the middlelayers.

The multilayer compact 914 can also be formed on a transport film, andthis transport film can be used, as is, as the aforementioned supportsubstrate 951, and can also be used as the support film 2.

The multilayer extrusion molding method and the die coater mentionedabove represent one example of the apparatus and method used forproducing the multilayer compact 914. Various other methods andapparatus, such as an injection molding method (apparatus), a coatingmethod (apparatus) or a printing method (apparatus) can also be used,provided they enable mixing of the compositions at the interfacesbetween the layers.

Next is a description of the polymer 915 and the additive 920.

(Polymer)

The polymer 915 is the material that constitutes the base polymer of theoptical waveguide 1.

It is preferable to use a polymer 915 having a satisfactorily high levelof transparency (which is transparent and colorless) and havingcompatibility with the monomer described below. In addition, it is alsopreferable to use a polymer 915 in which the monomer described below canreact (via a polymerization reaction or crosslinking reaction), andwhich retains satisfactory transparency even after polymerization of themonomer.

Here, the expression “having compatibility” means that the monomer canbe at least blended with the polymer 915 such that no phase separationwith the polymer 915 occurs within the optical waveguide-formingcompositions 901 and 902 or within the layer 910.

This type of polymer can be selected as required, and examples includeacrylic-based resins (polymers), methacrylic-based resins, cyclicether-based resins such as epoxy-based resins and oxetane-based resins,and polyolefin-based resins such as norbornene-based resins. One suchresin may be used alone, or a combination (such as a polymer alloy,polymer blend (mixture), or copolymer) of two or more resins may beused.

By using these types of resins as the polymer 915, an optical waveguide1 having excellent optical transmission properties can be obtained.

(Acrylic-based Polymer)

The acrylic-based polymer can be selected as required. For example, theuse of methyl (meth)acrylate, benzyl (meth)acrylate and/or cyclohexyl(meth)acrylate is preferable.

Further, examples of preferred raw material monomers for theacrylic-based polymer include MMA monomer (manufactured by Kuraray Co.,Ltd., or Mitsubishi Rayon Co., Ltd.).

The refractive index of each portion of the core layer 13 is determinedby the relative magnitude relationship between the refractive index ofthe (meth)acrylic-based polymer and the refractive index of the monomerand the content ratio between the polymer and the monomer in eachportion. Accordingly, by appropriate selection of the type of monomerused and the type of (meth)acrylic-based polymer used, the refractiveindex of each portion within the core layer 13 can be adjusted.

(Epoxy-based Polymer)

The epoxy-based polymer can be selected as required.

Epoxy-based polymers have particularly high levels of transparency andsuperior optical transmission properties, and also exhibit excellentheat resistance and adhesion, and can therefore be used favorably as thepolymer in the present invention. It is preferable to use an epoxy-basedpolymer having compatibility with the monomer described below, in whichthe monomer can react (via a polymerization reaction or crosslinkingreaction) in the manner described below, and which retains satisfactorytransparency even after reaction of the monomer.

The expression “having compatibility” means that the monomer can be atleast blended with the epoxy-based polymer such that no phase separationwith the epoxy-based polymer occurs within the optical waveguide-formingcompositions 901 and 902 or within the layer 910.

Moreover, an example of an alicyclic epoxy monomer is the compoundrepresented by formula (4) below.

The compound represented by formula (4) is3,4-epoxycyclohexenylmethyl-3′,4′-epoxycyclohexene carboxylate, and isavailable, for example, from Daicel Corporation as the product Celloxide2021P.

Further, in addition to the compound described above, the YP series ofphenoxy resins such as YP-50S or the product Ogsol EG (manufactured byOsaka Gas Chemicals Co., Ltd.) may also be used as the epoxy-basedpolymer or the raw material monomer.

The refractive index of each portion of the core layer 13 is determinedby the relative magnitude relationship between the refractive index ofthe epoxy-based polymer and the refractive index of the monomer, and thecontent ratio between the polymer and the monomer in each portion.Accordingly, by appropriate selection of the type of monomer used andthe type of epoxy-based polymer used, the refractive index of eachportion within the core layer 13 can be adjusted.

(Silicone-based Polymer)

Silicone-based polymers have particularly high levels of transparencyand superior optical transmission properties, and also exhibit excellentheat resistance, light stability and electrical insulation properties,and can therefore be used favorably as the polymer in the presentinvention. It is preferable to use a silicone-based polymer havingcompatibility with the monomer described below, and further preferablethat the silicone-based polymer can react (via a polymerization reactionor crosslinking reaction) with the monomer in the manner describedbelow, and can retain satisfactory transparency even after reaction ofthe monomer.

Here, the expression “having compatibility” means that the monomer canbe at least blended with the silicone-based polymer such that no phaseseparation with the silicone-based polymer occurs within the opticalwaveguide-forming compositions 901 and 902 or within the layer 910.

Moreover, a silicone-based polymer describes a polymer obtained bypolymerizing (via hydrolysis-condensation or condensation) a rawmaterial monomer composed of an organoalkoxysilane or a derivativethereof

Specific examples of the organoalkoxysilane includeisopropyltrimethoxysilane, neopentyltrimethoxysilane andallyltrimethoxysilane.

(Polyimide-based Polymer)

Polyimide-based polymers have particularly high levels of transparencyand superior optical transmission properties, and also exhibit excellentheat resistance, light stability, mechanical properties, adhesion andelectrical insulation properties. Accordingly, a polyimide-based polymercan be used favorably as the polymer in the present invention. It ispreferable to use a polyimide-based polymer having compatibility withthe monomer described below, and further preferable that thepolyimide-based polymer can react (via a polymerization reaction orcrosslinking reaction) with the monomer in the manner described below,and retains satisfactory transparency even after reaction of themonomer.

Here, the expression “having compatibility” means that when the monomeris blended with the polyimide-based polymer, no phase separation withthe polyimide-based polymer occurs within the optical waveguide-formingcompositions 901 and 902 or within the layer 910.

A polyimide-based polymer describes a polymer containing a polyimide(oligomer) prepared by heating and curing (imidizing) a polyamic acidobtained by reacting a tetracarboxylic anhydride with a diamine.

(Fluoropolymer)

Fluoropolymers have particularly high levels of transparency andsuperior optical transmission properties, and also exhibit excellentmechanical properties and moisture resistance. Accordingly, afluoropolymer can be used favorably as the polymer in the presentinvention. It is preferable to use a fluoropolymer having compatibilitywith the monomer described below, and further preferable that thefluoropolymer can react (via a polymerization reaction or crosslinkingreaction) with the monomer in the manner described below, and whichretains satisfactory transparency even after reaction of the monomer.

A fluoropolymer is a polymer containing fluorine atoms within themolecular structure. In the present invention, the fluoropolymer ispreferably a polymer having at least one type of ring structure selectedfrom among aliphatic ring structures, imide ring structures, triazinering structures, benzoxazole structures and aromatic ring structures,wherein the structure includes one or more fluorine atoms. Among suchpolymers, a polymer containing an aliphatic ring structure within themain chain is particularly desirable. This enables the layer 910obtained from the optical waveguide-forming compositions 901 and 902 toexhibit particularly uniform film thickness.

Examples of fluorine-containing aliphatic ring structure polymers thatcan be used favorably in the present invention include polymerscontaining structural units (repeating units) such as those shown belowin formulas (12) to (16) within the main chain.

In the above formulas, l is an integer of 0 to 5, m is an integer of 0to 4, and n is 0 or 1, provided that l+m+n is an integer of 1 to 6, eachof o, p and q independently represents an integer of 0 to 5, providedthat o+p+q is an integer of 1 to 6, each of R¹, R² and R³ independentlyrepresents F, Cl, CF₃, C₂F₅, C₃F₇ or OCF₃, and each of X¹ and X²independently represents F or Cl.(Polyolefin-based Polymer)

The polyolefin-based polymer can be selected as required.

The polyolefin-based polymer may also be a cyclic olefin-based polymersuch as a norbornene-based polymer or a benzocyclobutene-based polymer.For example, the polymers disclosed in Japanese Unexamined PatentApplication, First Publication No. 2000-090328 can be used as the cyclicolefin-based polymer.

(Additive)

In the present embodiment, in both the optical waveguide-formingcomposition 901 and the optical waveguide-forming composition 902, theadditive 920 includes a monomer. Further, in the present embodiment, theadditive 920 within the optical waveguide-forming composition 901 mayalso include a polymerization initiator, whereas on the other hand, theadditive 920 within the optical waveguide-forming composition 902 neednot include a polymerization initiator.

(Monomer)

The monomer (photopolymerizable monomer) is a compound which, uponirradiation with the active radiation described below, reacts within theirradiated region to form a reaction product, wherein the resultingdiffusion and migration of the monomer is able to generate a refractiveindex difference in the layer 910 between the irradiated region and thenon-irradiated region.

The reaction product of the monomer includes at least one of a polymerformed by polymerization of the monomer within the polymer 915, acrosslinked structure produced by crosslinking of molecules of thepolymer 915 via the monomer, and a branched structure produced bypolymerization of the monomer onto the polymer 915 to form a branchportion on the polymer 915.

The refractive index difference generated between the irradiated regionand the non-irradiated region occurs on the basis of the differencebetween the refractive index of the polymer 915 and the refractive indexof the monomer. Accordingly, the monomer contained within the additive920 is selected with due consideration of the magnitude relationship ofthe refractive index with that of the polymer 915.

Specifically, when it is desirable that the refractive index of theirradiated region increases in the layer 910, a combination of a polymer915 having a comparatively low refractive index and a monomer having ahigh refractive index compared with this polymer 915 is used. On theother hand, when it is desirable that the refractive index of theirradiated region decreases, a combination of a polymer 915 having acomparatively high refractive index and a monomer having a lowrefractive index compared with this polymer 915 is used.

The terms “high” and “low” used in relation to the refractive index donot refer to the absolute values of the refractive index, but ratherindicate the relative relationship of the refractive indices betweencertain materials.

When the refractive index of the irradiated region in the layer 910decreases as a result of the reaction of monomer (generation of thereaction product), the irradiated region corresponds with the lowrefractive index region WL of the refractive index distribution W. Whenthe refractive index of the irradiated region increases, the irradiatedregion corresponds with the high refractive index region WH of therefractive index distribution W.

A monomer having good compatibility with the polymer 915 and having adifference in refractive index compared with the polymer 915 of at least0.01 can be used favorably as the monomer.

This type of monomer may be any compound having a polymerizable sitewithin the molecular structure. The types of monomer mentioned above asthe raw material for the polymer 915 can be used, but there are noparticular limitations. Examples of the monomer include acrylic acid(methacrylic acid)-based monomers, epoxy-based monomers, oxetane-basedmonomers, norbornene-based monomers, vinyl ether-based monomers,styrene-based monomers and photodimerizable monomers. These monomers maybe used alone, or a combination of two or more monomers may be used.

Among these monomers, by using a monomer of a similar type to thepolymer 915, the monomer can be dispersed more uniformly within thepolymer 915. As a result, the properties of the opticalwaveguide-forming compositions 901 and 902 can be kept more uniform.

The molecular weight of the monomer may be selected as required. Forexample, the molecular weight is preferably from 50 to 500, morepreferably from 80 to 400, still more preferably from 100 to 400, andparticularly preferably from 100 to 350.

Furthermore, it is particularly preferable that the polymerizable siteof the monomer is an unsaturated hydrocarbon. Compounds containing anunsaturated hydrocarbon readily undergo polymerization reactions byradical polymerization and cationic polymerization, and are ideal as themonomer used in the present invention.

Examples of acrylic acid (methacrylic acid)-based monomers andepoxy-based monomers that can be used as the monomer include the samemonomers as those mentioned above as the raw material for the polymer915.

Further, in the case of monomers or oligomers having a cyclic ethergroup such as an oxetanyl group or epoxy group, because ring-opening ofthe cyclic ether group occurs readily, the monomer can react rapidly.Accordingly, by using such a monomer, the time required for formation ofthe core layer 13 can be shortened, enabling the production time for theoptical waveguide 1 to also be shortened.

The molecular weight of the monomer having a cyclic ether group or themolecular weight (weight-average molecular weight) of the oligomer maybe selected as required. For example, the molecular weight is preferablyfrom 50 to 500, more preferably from 80 to 400, still more preferablyfrom 100 to 400, and particularly preferably from 100 to 350.

There are no particular limitations on the combination of these types ofmonomers with the polymer 915, and any combination may be used.

An example of a monomer having an oxetanyl group that can be used is theproduct Aron Oxetane (manufactured by Toagosei Co., Ltd.).

Further, as mentioned above, at least a portion of the monomer may beoligomerized.

Examples of these monomers and oligomers having an oxetanyl group, andmonomers and oligomers having an epoxy group include compounds disclosedin Japanese Unexamined Patent Application, First Publication No.2010-090328.

The amount added of the monomer can be selected as required. The amountof the monomer is preferably at least 1 part by mass but not more than50 parts by mass, more preferably at least 2 parts by mass but not morethan 40 parts by mass, and still more preferably at least 15 parts bymass but not more than 40 parts by mass, per 100 parts by mass of thepolymer 915. This enables the variation in refractive index between thecore portions 14 and the side cladding portions 15 to be generated morereliably. Further, when forming the optical waveguide, the degree ofmigration of the monomer may also be adjusted by appropriate selectionof the method used. The degree of migration may also be controlled tocreate a preferred W type or GI type distribution in the transversewidth direction.

The monomers included in the optical waveguide-forming composition 901and the optical waveguide-forming composition 902 may have the samecomposition or different compositions.

Further, the optical waveguide-forming composition 901 may be configuredso as to contain a monomer, while the optical waveguide-formingcomposition 902 is configured without a monomer. In this case, monomerdiffusion and migration does not occur in each of the cladding layers 11and 12, and therefore a uniform refractive index can be achieved withineach of the cladding layers 11 and 12.

The photopolymerizable monomer described above is one type of so-calledrefractive index modifier. The refractive index modifier added as theadditive 920 may be a polymer or a monomer other than aphotopolymerizable monomer, provided the component has a differentrefractive index from that of the polymer 915. Examples of suchrefractive index modifiers include 2-bromotetrafluorobenzotrifluoride,chloropentafluorobenzene, decafluorobenzophenone, perfluoroacetophenone,perfluorobiphenyl and bromoheptafluoronaphthalene. Any one or more ofthese compounds may be used, or a mixture with other compounds may alsobe used.

When a refractive index modifier is used, the type of refractive indexdistribution described above can be formed by generating a concentrationgradient for the refractive index modifier. In order to form such aconcentration gradient, the refractive index modifier may be added tothe layer formed from the polymer 915 with appropriate variation in theamount added in accordance with the refractive index distribution thatis to be formed.

(Polymerization Initiator)

A polymerization initiator may be optionally included in thecomposition. The polymerization initiator is a material which acts uponthe monomer upon irradiation with the active radiation, therebypromoting the reaction of the monomer.

The polymerization initiator used is selected in accordance with thetype of polymerization reaction or crosslinking reaction of the monomer.For example, in the case of an acrylic acid (methacrylic acid)-basedmonomer or styrene-based monomer, a radical polymerization initiator canbe used favorably. In the case of an epoxy-based monomer, oxetane-basedmonomer or vinyl ether-based monomer, a cationic polymerizationinitiator can be used favorably.

Examples of the radical polymerization initiator include benzophenonesand acetophenones. Specific examples include Irgacure 651 and Irgacure184 (both manufactured by BASF Japan Ltd.).

On the other hand, examples of the cationic polymerization initiatorinclude Lewis acid generators such as diazonium salts, and Bronsted acidgenerators such as iodonium salts and sulfonium salts. Specific examplesinclude Adeka Optomer SP-170 (manufactured by Adeka Corporation), SanaidSI-100L (manufactured by Sanshin Chemical Industry Co., Ltd.), andRhodorsil 2074 (manufactured by Rhodia Japan Inc.).

The layer 910 containing the polymer 915 and the additive 920 describedabove has a prescribed refractive index due to the uniform dispersion ofthe additive 920 within the polymer 915.

Step [2]

Following the step [1], a mask (masking) 935 having openings (windows)9351 formed therein is prepared for the layer 910 formed in the mannerillustrated in FIG. 13(b), and the layer 910 is then irradiated withactive radiation 930 through this mask 935 (see FIG. 14).

Below is a description of one example in which a compound having a lowerrefractive index than that of the polymer 915 is used as the monomer.Further, in this regard, the composition of the polymer 915 is such thatthe optical waveguide-forming compositions 901 and 902 used in formingthe layer 910 are prepared so as to satisfy the relationship (refractiveindex of the optical waveguide-forming composition 901)>(refractiveindex of the optical waveguide-forming composition 902). As a result ofthese conditions, a refractive index distribution is formed in the layer910 in which the central portion in the thickness direction has thehighest refractive index, local minimum values exist between the centralportion and the upper surface and rear surface respectively of the layer910, and the refractive index changes in a continuous manner.

Furthermore, in the example illustrated here, the irradiated region 925that is irradiated with the active radiation 930 mainly becomes the sidecladding portions 15.

Accordingly, in the example illustrated here, openings (windows) 9351equivalent to, namely having the same shape as or a substantiallysimilar shape to, the pattern of the side cladding portions 15 that areto be formed are formed in the mask 935. These openings 9351 havetransmission portions through which the irradiated active radiation 930travels. The pattern within the core portions 14 and the side claddingportions 15 is determined on the basis of the refractive indexdistribution W formed in accordance with the irradiation of the activeradiation 930. As a result, the pattern of the openings 9351 and thepattern of the side cladding portions 15 are not necessarily exactly thesame, and some minor differences may occur between the patterns.

The mask 935 may be formed in advance (in a separate preparation) (suchas a plate-like mask), or may be formed on the layer 910 by a vapordeposition method or coating method or the like.

Preferred examples of the mask 935 include photomasks made of quartzglass or a PET base material or the like, stencil masks, and metal thinfilms formed using a vapor phase deposition method (such as a vapordeposition method or sputtering method or the like). Among thesepossibilities, the use of a photomask or stencil mask is particularlydesirable. This is because such masks enable a fine pattern to be formedwith high precision, and also provide ready handling, which isadvantageous in improving productivity.

Further, in FIG. 14, the openings (windows) 9351 of the mask 935 areformed by partially removing the mask in accordance with the pattern ofthe irradiated region 925 of the active radiation 930. When a photomaskmade of quart glass or a PET base material is used, the mask can also beobtained by providing, in the appropriate locations on the photomask,shielding portions for the active radiation 930 formed from a shieldingmaterial composed of metal such as chromium. In such a mask, thoseportions on which the shielding portions are not formed act as thewindows (transmission portions).

The active radiation 930 used may be any radiation capable of causing aphotochemical reaction (change) in the polymerization initiator. Forexample, visible light, ultraviolet light, infrared light, laser light,or an electron beam or X-rays can be used.

There are no particular limitations on the active radiation 930, whichmay be selected appropriately in accordance with the polymerizationinitiator and the like. Active radiation having a peak wavelength in thewavelength range from 200 to 450 nm is preferable. This enables thepolymerization initiator to be activated comparatively easily.

When the active radiation 930 is irradiated through the mask 935 andonto the layer 910, the monomer polymerizes in an irradiated region 9253in the core layer 13, which represents a portion of the irradiatedregion 925. As a result of the monomer polymerization, the amount of themonomer in the irradiated region 9253 decreases. Consequently, themonomer within a non-irradiated region 9403 in the core layer, whichrepresents a portion of the non-irradiated region 940, diffuses andmigrates into the irradiated region 9253. As described above, thepolymer 915 and the monomer are selected appropriately so as to exhibita difference in refractive index. Accordingly, the diffusion andmigration of the monomer is accompanied by the generation of arefractive index difference between the irradiated region 9253 and thenon-irradiated region 9403 of the core layer 13. On the other hand, inirradiated regions 9251 and 9252 within the cladding layers 11 and 12,by employing conditions wherein the optical waveguide-formingcomposition 902 does not contain a monomer, the polymerization reactionof the monomer is suppressed.

In the cladding layers 11 and 12, the method used for suppressing themonomer polymerization reaction within the irradiated regions 9251 and9252 can be selected as appropriate. For example, a method may beemployed in which the type of monomer is changed, or alternatively, thepolymerization initiator may be added to the additive 920 in the opticalwaveguide-forming composition 901, but either excluded from, or onlyincluded in a small amount within the additive 920 of the opticalwaveguide-forming composition 902. In this case, in the core layer 13,the irradiation means that the monomer is able to benefit from theaction of the polymerization initiator, and therefore undergoessatisfactory polymerization and migration. In contrast, in the claddinglayers, because the polymerization initiator is either absent or presentin only a small amount, the monomer undergoes little or nopolymerization, and migration of the monomer is also minimal ornon-existent.

FIG. 16 is a schematic diagram describing the generation of a refractiveindex difference between the irradiated region 9253 and thenon-irradiated region 9403 of the core layer 13 illustrated in FIG. 14.The diagram illustrates the change in the refractive index distributiondue to the irradiation, with the position in the width direction of thetransverse cross-section shown along the horizontal axis, and therefractive index of the transverse cross-section shown along thevertical axis.

In the present embodiment, a monomer having a lower refractive indexthan that of the polymer 915 is used. As a result, the dispersion andmigration of the monomer is accompanied by an increase in the refractiveindex of the non-irradiated region 9403, and a decrease in therefractive index of the irradiated region 9253 (see FIG. 16). It isthought that because the width of the irradiated portion and the widthof the non-irradiated portion are narrow, the monomer is able to migrateadequately.

It is thought that the diffusion and migration of the monomer occurs asa result of the monomer concentration gradient that is formed due to theconsumption of the monomer in the irradiated region 9253. Accordingly,the monomer within the entire non-irradiated region 9403 does notmigrate simultaneously toward the irradiated region 9253, but rather themigration begins gradually from the monomer located close to theirradiated region 9253, and migration of the monomer from the centralregion of the non-irradiated region 9403 toward the outside of theregion then occurs in order to replenish the monomer concentration. As aresult, as illustrated in FIG. 16, a high refractive index portion H andlow refractive index portions L are formed on either side of theboundaries between the irradiated regions 9253 and the non-irradiatedregion 9403, with the high refractive index portion H positioned on theside of the non-irradiated region 9403 and the low refractive indexportions L positioned on the sides of the irradiated regions 9253. Thishigh refractive index portion H and these low refractive index portionsL are formed due to the diffusion and migration of the monomer. As aresult, they necessarily adopt smooth curves. Specifically, the highrefractive index portion H adopts, for example, a substantially convexinverted U-shape, and the low refractive index portions L adopt asubstantially concave U-shape.

The refractive index of the polymer formed when the type of monomerdescribed above polymerizes is substantially the same as the refractiveindex of the monomer prior to polymerization (with a difference in therefractive index of about 0 to 0.001). Consequently, in the irradiatedregion 9253, as the polymerization of the monomer proceeds, the decreasein the refractive index proceeds in accordance with the amount ofmigrating monomer and the amount of monomer-derived material.Accordingly, by appropriate adjustment of the amount of the monomerrelative to the polymer, and the amount of the polymerization initiatorand the like, the shape of the refractive index distribution W can becontrolled. For example, either the distribution illustrated in FIG. 3or the distribution illustrated in FIG. 4 can be freely selected.

On the other hand, in the non-irradiated region 9403, because thepolymerization initiator is not activated, polymerization of the monomeris not promoted.

By adjusting the irradiation dose of the active radiation 930, therefractive index difference that is formed and the shape of therefractive index distribution can be controlled. For example, byincreasing the irradiation dose, the refractive index difference can beincreased. Further, the layer 910 may be dried prior to the irradiationwith the active radiation 930. In such a case, the shape of therefractive index distribution can also be controlled by adjusting thedegree of drying. For example, by increasing the degree of drying, thediffusion and migration of the monomer can be suppressed. For example,either the distribution illustrated in FIG. 3 or the distributionillustrated in FIG. 4 can be freely selected.

Further, in the irradiated region 9253, diffusion and migration of themonomer may occur not only from the non-irradiated region 9403 in thecore layer 13, but also from the irradiated region 9251 in the claddinglayer 11 and the irradiated region 9252 in the cladding layer 12, whichrepresent portions of the irradiated region 9253. If the refractiveindex of the monomer contained in the cladding layers is also low, thenthis diffusion and migration from the cladding layers will cause an evengreater decrease in the refractive index in the irradiated region 9253.On the other hand, in the irradiated region 9251 and the irradiatedregion 9252, the diffusion and migration of the monomer is accompaniedby an increase in the refractive index, but the composition and the likeof the polymer 915 in these regions is set so that the refractive indexis low. Accordingly, even if the refractive index increases, it does notimpair the functions of the optical waveguide 1.

The optical waveguide 1 is obtained using the principles described above(see FIG. 15). This type of optical waveguide 1 has a refractive indexdistribution W, which exhibits variation in the refractive index withinthe layer 910 formed from an optical waveguide-forming compositionprepared by dispersing a monomer in the polymer 915, the refractiveindex distribution W being formed by irradiating the active radiation930 partially onto the layer 910, thereby causing diffusion andmigration of the monomer, leading to uneven distribution of the monomer.Further, this type of optical waveguide 1 can be formed simply by thepartial irradiation of the active radiation 930, and yields an opticalwaveguide having high transmission efficiency. Accordingly, even if thewidths and pitch of the core portions 14 and the side cladding portions15 are narrow, high quality optical communication can still be achieved.Further, even if a plurality of core portions 14 are intersected withinthe same plane, interference or deterioration in the transmissionefficiency are unlikely. Accordingly, the optical waveguide lfacilitates the production of multichannel and high-densityconfigurations.

In the refractive index distribution W, the local minimum values Ws1,Ws2, Ws3 and Ws4 exist at positions where the low refractive indexportion L inverts (see FIG. 3(b)), and the positions of these localminimum values correspond with the interfaces between the core portions14 and the side cladding portions 15.

Further, when a compound having a higher refractive index than that ofthe polymer 915 is used as the monomer, then in the opposite manner tothat described above, the diffusion and migration of the monomer isaccompanied by an increase in the refractive index of the migrationdestination. Accordingly, the irradiated region 925 and thenon-irradiated region 940 must be set in accordance with that behavior.

As described above, the refractive index distribution W is formed with acontinuously changing refractive index, due to the migration and unevendistribution of the photopolymerizable monomer. Consequently, the corelayer 13 does not have distinct structural interfaces between the coreportions 14 and the side cladding portions 15. As a result, problemssuch as peeling and cracking are unlikely, and the optical waveguide 1exhibits excellent reliability.

The Shore D hardness of the polymer 915 is preferably about 35 to 95,more preferably about 40 to 90, and still more preferably about 45 to85. A polymer 915 having this level of hardness imparts the opticalwaveguide 1 with the necessary flexibility and rupture resistance, whilealso enabling reliable diffusion and migration of the monomer, thuscontributing to the formation of a satisfactory refractive indexdifference. Accordingly, the obtained optical waveguide 1 is providedwith the necessary flexibility and mechanical strength suitable forbending, and has excellent optical properties even in a bent state.

Similarly, the Rockwell hardness of the polymer 915 is preferably about40 to 125 on the M scale, more preferably about 50 to 115, and stillmore preferably about 60 to 110.

Furthermore, the softening point of the polymer 915 is preferably atleast 90° but not more than 300° C., more preferably from 95 to 280° C.,and particularly preferably from 100 to 260° C. This ensures that therefractive index distribution W can be formed reliably in the resultingoptical waveguide 1, that the formed refractive index distribution W canbe reliably maintained over long periods, and that the optical waveguide1 has sufficient mechanical strength to prevent disconnection even whenthe optical waveguide is used in a bent state. Accordingly, the opticalwaveguide 1 becomes a highly reliable device with excellent opticalproperties. The softening point of the polymer 915 refers to the glasstransition temperature or the melting point of the polymer 915, and ifthe polymer exhibits both values, refers to the lower of the two values.

In the intersection portion 147, the monomer diffuses and migrates infour directions, and the width of the refractive index variationincreases. In this manner, the local maximum value of the refractiveindex at the intersection portion 147 can be increased to a higher valuethan the local maximum value in the core portion 14.

On the other hand, the layers 914 a to 914 i exist in the layer 910prior to irradiation with the active radiation 930, and therefore asillustrated in FIG. 17A, a refractive index distribution T′ is formed inthe thickness direction of the layer 910. In other words, a refractiveindex distribution (W type) is formed with the refractive index alongthe horizontal axis and the position (distance) in the thicknessdirection of the transverse cross-section along the vertical axis.

This refractive index distribution T′ is formed in the manner describedabove, by using the optical waveguide-forming composition 901 and theoptical waveguide-forming composition 902 having mutually differentrefractive indices to form the layer 910 using a multilayer moldingmethod. If two of these structures are superimposed, then the type ofrefractive index distribution illustrated in FIG. 10(b) can also beobtained.

When the layer 910 is irradiated with the active radiation 930 throughthe mask 935, in those cases where there is a difference in monomercontent between the optical waveguide-forming composition 901 and theoptical waveguide-forming composition 902, the monomer within thenon-irradiated region 9403 diffuses and migrates into the irradiatedregion 9253, and therefore in the refractive index distribution T′ inthe thickness direction of the core portion 14, the refractive index ofthe region corresponding with the core portion 14 increases. On theother hand, in the cladding layers 11 and 12 positioned below and abovethe core portion 14, the refractive index either does not change orchanges only minimally, and as a result, the refractive index differencebetween the core portion 14 and the lower and upper cladding layers 11and 12 is magnified.

Based on the above principles, an optical waveguide 1 having arefractive index distribution T with a large difference in refractiveindex between the local minimum values and the local maximum value canbe obtained (see FIG. 17B). In those cases where the refractive indexdistribution T′ already has a refractive index distribution shapesufficient to produce satisfactory effects, the above-describedconversion from the refractive index distribution T′ to the refractiveindex distribution T may be omitted.

In FIG. 13(a), if the uppermost layer and the lowermost layer formedfrom the optical waveguide-forming composition 901 are not provided,then an optical waveguide having the type of thickness directionrefractive index distribution illustrated in FIG. 17C can be obtained.

The refractive index distribution W has a fixed correlation with theconcentration of monomer-derived structures within the core layer 13.Accordingly, by measuring the concentration of these monomer-derivedstructures or the concentration of the materials of those structures,the refractive index distribution W of the optical waveguide 1 can beidentified indirectly.

Similarly, the refractive index distribution T has a fixed correlationwith the concentration of monomer-derived structures, or theconcentration of the materials of those structures, within the opticalwaveguide 1. Accordingly, by measuring the concentration of thesemonomer-derived structures, the refractive index distribution T of theoptical waveguide 1 can be identified indirectly.

The expression “monomer-derived structures” refers to the unreactedmonomer and structures formed as a result of reaction, and includes themonomer, oligomers formed by reaction of the monomer, and polymersformed by reaction of the monomer.

Measurement of the concentration of these structures can be conducted bylinear analysis such as FT-IR or TOF-SIMS, or by area analysis or thelike.

Moreover, the refractive index distribution W and the refractive indexdistribution T can also be determined indirectly by utilizing the factthat the intensity distribution of output light from the opticalwaveguide 1 has a fixed correlation with the refractive indexdistribution W or the refractive index distribution T. In other words,the intensity distribution of the output light may be used as theconcentration measurement.

Furthermore, the refractive index distribution can be measured by (1) amethod in which interference fringes, which are dependent on therefractive index, are measured using a dual-beam interferencemicroscope, and the refractive index distribution is then calculatedfrom these interference fringes, or (2) a method in which thedistribution is measured directly using the refracted near field (RNF)method. Of these, the refracted near field method may employ, forexample, the measurement conditions disclosed in Japanese UnexaminedPatent Application, First Publication No. Hei 05-332880. On the otherhand, the dual-beam interference microscope is preferred in terms ofenabling relatively simple measurement of the refractive indexdistribution.

One example of the procedure for measuring the refractive indexdistribution using a dual-beam interference microscope is describedbelow. First, the optical waveguide is sliced in the cross-sectionaldirection (width direction) to obtain an optical waveguide slice. Forexample, slicing is performed so as to obtain an optical waveguidelength of 200 to 300 μm. Subsequently, a chamber is prepared by fillingthe space surrounded by two slide glass sheets with an oil having arefractive index of 1.536. Then, a measurement sample containing theoptical waveguide slice sandwiched inside the internal space of thechamber, and a blank sample containing no optical waveguide slice areprepared. Next, using the dual-beam interference microscope, light splitinto two beams is irradiated onto the measurement sample and the blanksample respectively, and the transmitted light is combined to obtain aninterference fringe photograph (see FIG. 23, an interference fringephotograph of an optical waveguide of the present invention, having atransverse W type distribution and a longitudinal GI type distribution).The interference fringes are generated in accordance with the refractiveindex distribution (phase distribution) of the optical waveguide slice.Accordingly, by performing image analysis of the obtained interferencefringe photograph, the refractive index distribution W in the widthdirection of the optical waveguide can be obtained (see FIG. 21, a Wtype refractive index distribution in the width direction of an opticalwaveguide of the present invention, having a transverse W type andlongitudinal SI type distribution). When acquiring the refractive indexdistribution W, performing image analysis of a plurality of interferencefringe photographs can enhance the accuracy of the refractive indexdistribution W. In order to obtain a plurality of interference fringephotographs, the prism inside the dual-beam interference microscope maybe moved, thereby altering the optical path length and obtainingphotographs having mutually different intervals between the interferencefringes or having different positions in which the interference fringesare formed. Further, when performing image analysis of the interferencefringe photographs, analysis points may be set, for example, at aninterval of 2.5 μm. By performing this image analysis, the refractiveindex distribution W in the width direction and the refractive indexdistribution T in the thickness direction can be obtained for theoptical waveguide, and therefore a three dimensional refractive indexdistribution for the optical waveguide of the present invention, such asthat illustrated in FIG. 22, can be obtained. The optical waveguide 1 isobtained in the manner described above.

Subsequently, if required, the optical waveguide 1 is detached from thesupport substrate 951, the support film 2 is laminated to the lowersurface of the optical waveguide 1, and the cover film 3 is laminated tothe upper surface.

The optical waveguide of the present invention preferably has a W typeor GI type distribution shape in cross-section (transversecross-section). Further, the optical waveguide of the present inventionpreferably has a W type, GI type or SI type distribution shape incross-section (cross-section of the thickness direction).

Among the various possibilities, the optical waveguide 1 preferably hasa W type distribution shape in the transverse cross-section and a GItype or SI type distribution shape in the cross-section of the thicknessdirection. An optical waveguide having a W type distribution shape inthe transverse cross-section and a GI type distribution shape in thecross-section of the thickness direction is particularly desirable.

The graphs of FIG. 18 to FIG. 20 illustrate the transmission loss at anintersection portion between two core portions. Specifically, thehorizontal axis represents the number of intersections or crossings, andthe vertical axis represents the relative loss compared with the case inwhich there are no intersections. The intersection portions of the coreportions were formed by a method using a photomask in which theintersection portions were formed as a pattern. The crossing angle ofthe core portions in the figures is 90°, 60° and 30° respectively.

These graphs illustrate the results for two waveguides formed undersubstantially the same conditions, specifically (1) an optical waveguidehaving a W type distribution shape in the transverse cross-section andan SI type distribution shape in the cross-section of the thicknessdirection (shown as black squares), and (2) an optical waveguide havinga W type distribution shape in the transverse cross-section and a GItype distribution shape in the cross-section of the thickness direction(shown as white squares). The straight line shown in FIG. 18 is a linecalculated by assuming a loss of 0.02 dB per intersection.

Further, the data shown as crosses (x) represents data for an opticalwaveguide formed using conventional technology, and is plotted for thepurpose of reference. The production conditions differ, and thereforethe data is graphed merely to provide a point of reference. The data forthe conventional technology is reproduced, as is, from the document“Optical interconnection using VCSELs and polymeric waveguide circuits”,T. Sakamoto, H. Tsuda, M. Hikita, T. Kagawa, K. Tateno, and C. Amano, J.Lightwave Technol. 11, 1487 to 1492 (2000).

As illustrated in FIG. 18 to FIG. 20, the optical waveguide of thepresent invention having a combination of a W type distribution and anSI type distribution, and the optical waveguide having a combination ofa W type distribution and a GI type distribution both exhibit excellentlow levels of transmission loss. In particular, the optical waveguidehaving a combination of a W type distribution and a GI type distributionis able to achieve extremely low levels of transmission loss at all ofthe evaluated crossing angles.

Furthermore, when the layer 910 is formed so as to include a pluralityof core layers 13, and the active radiation 930 is irradiated onto thislayer 910, a single irradiation enables the simultaneous formation ofthe core portions 14 and the side cladding portions 15 in this pluralityof core layers 13. As a result, an optical waveguide 1 having aplurality of core layers 13 can be produced via a minimal number ofsteps. Further, in this case, almost no positional deviation of the coreportions 14 occurs between the plurality of core layers 13. Accordingly,an optical waveguide 1 having extremely high dimensional precision canbe obtained. This type of optical waveguide 1 exhibits particularlysuperior optical coupling efficiency when optically coupled with lightreceiving and emitting elements.

Furthermore, the optical waveguide of the present invention exhibitslittle transmission loss or pulse signal rounding, and if a multichannelor high density configuration is produced, crosstalk or interference atthe intersection portions is unlikely. As a result, even at high densityand within a small surface area, an optical waveguide of highreliability can be obtained, and by installing this type of opticalwaveguide, electronic devices of improved reliability and betterminiaturization can be obtained.

An optical waveguide, optical wiring component, optical waveguide moduleand electronic device of the present invention have been describedabove, but the present invention is not limited to the configurationsdescribed above, and for example, optional structures may be added tothe optical waveguide.

Further, the method used for producing the optical waveguide of thepresent invention is not limited to the method described above. Forexample, other methods may also be used, including a method in which theirradiation of active radiation is used to break molecular bonds, thusaltering the refractive index (photo-bleaching method), and a method inwhich a photo-crosslinkable polymer having an unsaturated bond that canundergo photoisomerization or photodimerization is added to thecomposition used in forming the core layer, and irradiation of activeradiation is used to change the molecular structure and thereby alterthe refractive index (photoisomerization or photodimerization method).

With these methods, the amount of change in the refractive index can beadjusted in accordance with the irradiation dose of the activeradiation. Accordingly, by altering the dose of active radiationirradiated onto each portion of the layer in accordance with thetargeted shape of the refractive index distribution W, a core layerhaving the desired refractive index distribution W can be formed.

EXAMPLES

Next is a description of examples of the present invention. However, thepresent invention is not limited to only these examples. Modifications,additions or substitutions or the like of positions, numbers, amountsand varieties and the like may be made, provided they result in noparticular problems.

1. Production of Optical Waveguide Having Refractive Index DistributionShown in FIG. 3

First, optical waveguides having linear core portions with therefractive index distribution illustrated in FIG. 3 were produced undervarious conditions (Examples 1 to 18), and for the purposes ofcomparison, optical waveguides of Comparative Example 1 and ReferenceExamples 1 to 4 were also produced. These optical waveguides wereevaluated as described in Section 3 below.

Example 1

(1) Production of Cladding Layer-forming Resin Composition

Twenty grams of an alicyclic epoxy resin Celloxide 2081 manufactured byDaicel Corporation, 0.6 g of a cationic polymerization initiator AdekaOptomer SP-170 manufactured by Adeka Corporation, and 80 g of methylisobutyl ketone were mixed under stirring to prepare a solution.

Subsequently, the thus obtained solution was filtered through a PTFEfilter with a pore size of 0.2 μm, thus obtaining a clean, colorless andtransparent cladding layer-forming resin composition E1.

(2) Production of Photosensitive Resin Composition

Twenty grams of a phenoxy resin YP-50S manufactured by Nippon Steel andChemical Co., Ltd. as an epoxy-based polymer, 5 g of Celloxide 2021Pmanufactured by Daicel Corporation as a monomer, and 0.2 g of AdekaOptomer SP170 manufactured by Adeka Corporation as a polymerizationinitiator were added to 80 g of methyl isobutyl ketone and dissolvedunder stirring to prepare a solution.

Subsequently, the thus obtained solution was filtered through a PTFEfilter with a pore size of 0.2 μm, thus obtaining a clean, colorless andtransparent photosensitive resin composition F1.

(3) Preparation of Lower Cladding Layer

The cladding layer-forming resin composition E1 was applied uniformly toa polyimide film of thickness 25 μm using a doctor blade. The resultingstructure was then placed in a dryer at 50° C. for 10 minutes. Followingcomplete removal of the solvent, the entire surface of the structure wasirradiated with ultraviolet radiation using a UV exposure apparatus,thereby curing the applied resin composition E1. As a result, acolorless and transparent lower cladding layer having a thickness of 10μm was obtained. The accumulated dose of ultraviolet radiation was 500mJ/cm².

(4) Preparation of Core Layer

The photosensitive resin composition F1 was applied uniformly to theprepared lower cladding layer using a doctor blade. The resultingstructure was then placed in a dryer at 40° C. for 5 minutes. Followingcomplete removal of the solvent and formation of a film, a photomaskhaving a linear pattern of lines and spaces over the entire surface waspressed onto the obtained film. The film was then irradiated withultraviolet radiation from above the photomask using a parallel exposureapparatus. The accumulated dose of ultraviolet radiation was 1,000mJ/cm².

Subsequently, the photomask was removed, and the structure was placed inan oven at 150° C. for 30 minutes. Upon removal from the oven, theappearance of a clear waveguide pattern in the film was confirmed. Theaverage width WCO of the core portions and the average width WCL of theside cladding portions are shown in Table 1. The obtained core layer hada thickness of 50 μm, and included 8 core portions.

(5) Preparation of Upper Cladding Layer

In a similar manner to (3) above, the cladding layer-forming resincomposition E1 was applied to the prepared core layer to obtain acolorless and transparent upper cladding layer with a thickness of 10μm. In this manner, an optical waveguide was obtained.

(6) Evaluation of Refractive Index Distribution

Using a dual-beam interference microscope, a refractive indexdistribution W in the width direction was acquired for a transversecross-section of the core layer of the obtained optical waveguide. Theresults revealed that the refractive index distribution W had aplurality of low refractive index regions and high refractive indexregions, with the refractive index changing in a continuous manner.

Examples 2 to 8

With the exceptions of setting the polymer composition, the monomercomposition and content, and the accumulated dose of ultravioletradiation as shown in Table 1, and preparing the photomask pattern sothat the average width WCO of the core portions and the average widthWCL of the side cladding portions exhibited the values shown in Table 1,optical waveguides of Examples 2 to 8 were obtained in the same manneras Example 1.

Example 9

(1) Synthesis of (Meth)acrylic-based Polymer

A separable flask was charged with 20.0 g of methyl methacrylate (MMA),30.0 g of benzyl methacrylate (BzMA) and 450 g of methyl isobutylketone. These components were stirred and mixed, and the flask wasflushed with nitrogen, thus obtaining a monomer solution.

On the other hand, 0.25 g of azobisisobutyronitrile as a polymerizationinitiator was dissolved in 10 g of methyl isobutyl ketone, and the flaskwas flushed with nitrogen to obtain an initiator solution.

The aforementioned monomer solution was stirred under heating at 80° C.,and a syringe was used to add the above initiator solution to themonomer solution. The mixture was stirred at 80° C. for one hour, andthen cooled to complete preparation of a polymer solution. Subsequently,5 L of isopropanol was placed in a beaker, and with the isopropanolundergoing constant stirring with a stirrer at normal temperature, thepolymer solution was added dropwise to the beaker. Following completionof the dropwise addition, stirring was continued for a further 30minutes, and the precipitated polymer was then collected and dried in avacuum dryer under reduced pressure at 60° C. for 8 hours. This yieldedan acrylic-based polymer A1.

(2) Production of Cladding Layer-forming Resin Composition

Twenty grams of a water-based acrylate resin solution RD-180manufactured by Goo Chemical Co. Ltd., 20 g of isopropanol, and 0.4 g ofCarbodilite V-02-L2 manufactured by Nisshinbo Chemical Inc. as apolymerization initiator were mixed under stirring to prepare asolution.

Subsequently, the thus obtained solution was filtered through a PTFEfilter having a pore size of 0.2 μm, thus obtaining a clean, colorlessand transparent cladding layer-forming resin composition B1.

(3) Production of Photosensitive Resin Composition

Twenty grams of the synthesized acrylic-based polymer A1, 5 g ofcyclohexyl methacrylate as a monomer, and 0.2 g of Irgacure 651manufactured by BASF Japan Ltd. as a polymerization initiator were addedto 80 g of methyl isobutyl ketone and dissolved under stirring toprepare a solution.

Subsequently, the thus obtained solution was filtered through a PTFEfilter having a pore size of 0.2 μm, thus obtaining a clean, colorlessand transparent photosensitive resin composition C1.

(4) Preparation of Lower Cladding Layer

The cladding layer-forming resin composition B1 was applied uniformly toa polyimide film of thickness 25 μm using a doctor blade. The resultingstructure was then placed in a dryer at 80° C. for 10 minutes. Followingcomplete removal of the solvent, the structure was placed in an oven at150° C. for 10 minutes to cure the composition, thus obtaining acolorless and transparent lower cladding layer having a thickness of 10μm.

(5) Preparation of Core Layer

The photosensitive resin composition C1 was applied uniformly to theprepared lower cladding layer using a doctor blade. The resultingstructure was then placed in a dryer at 40° C. for 5 minutes. Followingcomplete removal of the solvent and formation of a film, a photomaskhaving a linear pattern of lines and spaces over the entire surface waspressed onto the obtained film. The film was then irradiated withultraviolet radiation from above the photomask using a parallel exposureapparatus. The accumulated dose of ultraviolet radiation was 800 mJ/cm².

Subsequently, the photomask was removed, and the structure was placed inan oven at 150° C. for 30 minutes. Upon removal from the oven, theappearance in the film of a distinct waveguide pattern with arectangular cross-section was confirmed. The average width WCO of thecore portions and the average width WCL of the side cladding portionsare shown in Table 2. The obtained core layer had a thickness of 50 μm,and included 8 core portions.

(6) Preparation of Upper Cladding Layer

In a similar manner to (4) above, the cladding layer-forming resincomposition B1 was applied to the prepared core layer to obtain acolorless and transparent upper cladding layer with a thickness of 10μm. In this manner, an optical waveguide was obtained.

(7) Evaluation of Refractive Index Distribution

Using a dual-beam interference microscope, a refractive indexdistribution W in the width direction was acquired for a transversecross-section of the core layer of the obtained optical waveguide. Theresults revealed that the refractive index distribution W had aplurality of low refractive index regions and high refractive indexregions, with the refractive index changing in a continuous manner.

Examples 10 to 12

With the exceptions of setting the monomer composition and content, andthe accumulated dose of ultraviolet radiation as shown in Table 2, andpreparing the photomask pattern so that the average width WCO of thecore portions and the average width WCL of the side cladding portionsexhibited the values shown in Table 2, optical waveguides of Examples 10to 12 were obtained in the same manner as Example 9.

Example 13

(1) Synthesis of Polyolefin-based Resin

In a glove box filled with dry nitrogen, in which the moisture contentand oxygen concentration were both suppressed to 1 ppm or lower, 7.2 g(40.1 mmol) of hexylnorbornene (HxNB) and 12.9 g (40.1 mmol) ofdiphenylmethyl norbornene methoxysilane were weighed into a 500 mL vial,and 60 g of dehydrated toluene and 11 g of ethyl acetate were then addedto the vial. The glove box was tightly sealed at the top by a siliconesealer.

Next, 1.56 g (3.2 mmol) of a Ni catalyst and 10 mL of dehydrated toluenewere weighed into a 100 mL vial. A stirrer chip was placed in the vial,the vial was then sealed, and the catalyst was completely dissolved bythorough stirring.

One mL of this Ni catalyst solution was measured accurately using asyringe and injected quantitatively into the vial containing the abovetwo types of norbornene compounds, and the resulting mixture was stirredat room temperature for one hour. As a result, a marked increase in theviscosity was confirmed. At this point, the seal was removed and 60 g oftetrahydrofuran (THF) was added and stirred, thus obtaining a reactionsolution.

A 100 mL beaker was charged with 9.5 g of acetic anhydride, 18 g ofaqueous hydrogen peroxide (concentration: 30%), and 30 g of deionizedwater, and the mixture was stirred to prepare an aqueous peracetic acidsolution in situ. Next, the entire volume of this aqueous solution wasadded to the above reaction solution and stirred for 12 hours to effecta Ni reduction treatment.

Next, following completion of the above treatment, the reaction solutionwas transferred to a separating funnel, and the lower aqueous layer wasdiscarded. Subsequently, 100 mL of a 30% aqueous solution of isopropanolwas added and shaken vigorously. The contents were left to stand toallow complete separation into two layers, and the aqueous layer wasthen discarded. This water washing process was repeated a total of threetimes. The oil layer was then added dropwise to a large excess ofacetone to re-precipitate the produced polymer, and the polymer wasseparated from the filtrate by filtration. Subsequently, the polymer wasdried by heating for 12 hours in a vacuum dryer set to 60° C., thusobtaining a polymer #1. Measurement of the molecular weight distributionof the polymer #1 by GPC revealed Mw=100,000 and Mn=40,000. The molarratio between each of the structural units in the polymer #1 wasdetermined by NMR, and the results revealed 50 mol % of hexylnorbornenestructural units and 50 mol % of diphenylmethyl norbornene methoxysilanestructural units.

(2) Production of Core Layer-forming Composition

Ten grams of the purified polymer #1 was weighed into a 100 mL glasscontainer, and then 40 g of mesitylene, 0.01 g of an antioxidant Irganox1076 (manufactured by Ciba Geigy Ltd.), 2 g of cyclohexyloxetane monomer(CHOX, manufactured by Toagosei Co. Ltd., CAS #483303-25-9, molecularweight: 186, boiling point: 125° C./1.33 kPa) and a polymerizationinitiator (photoacid generator) Rhodorsil Photoinitiator 2074(manufactured by Rhodia Inc., CAS #178233-72-2) (0.0125 g in 0.1 mL ofethyl acetate) were added to the container and dissolved uniformly.Subsequently, the solution was filtered through a 0.2 μm PTFE filter toobtain a clean core layer-forming composition. In Table 1, the abovepolymerization initiator is shown as PI 2074.

(3) Production of Cladding Layer-forming Composition

With the exception of replacing the above polymer #1 with a polymer inwhich the molar ratio between each of the structural units in thepurified polymer #1 had been altered to obtain a polymer containing 80mol % of the hexylnorbornene structural units and 20 mol % of thediphenylmethyl norbornene methoxysilane structural units, a claddinglayer-forming composition was obtained in the same manner as the corelayer-forming composition.

(4) Preparation of Lower Cladding Layer

The cladding layer-forming composition was applied uniformly to apolyimide film of thickness 25 μm using a doctor blade. The resultingstructure was then placed in a dryer at 50° C. for 10 minutes. Followingcomplete removal of the solvent, the entire surface of the structure wasirradiated with ultraviolet radiation using a UV exposure apparatus,thereby curing the applied resin composition. As a result, a colorlessand transparent lower cladding layer having a thickness of 10 μm wasobtained. The accumulated dose of ultraviolet radiation was 500 mJ/cm².

(5) Preparation of Core Layer

The core layer-forming composition was applied uniformly to the preparedlower cladding layer using a doctor blade. The resulting structure wasthen placed in a dryer at 40° C. for 5 minutes. Following completeremoval of the solvent and formation of a film, a photomask having alinear pattern of lines and spaces over the entire surface was pressedonto the obtained film. The film was then irradiated with ultravioletradiation from above the photomask using a parallel exposure apparatus.The accumulated dose of ultraviolet radiation was 1,300 mJ/cm².

Subsequently, the photomask was removed, and the structure was placed inan oven at 150° C. for 30 minutes. Upon removal from the oven, theappearance in the film of a distinct waveguide pattern with arectangular cross-section was confirmed. The obtained core layer had athickness of 50 μm. Further, the core layer included 8 core portions.

(6) Preparation of Upper Cladding Layer

In a similar manner to (3) above, the cladding layer-forming resincomposition E1 was applied to the prepared core layer to obtain acolorless and transparent upper cladding layer with a thickness of 10μm. In this manner, an optical waveguide was obtained.

(7) Evaluation of Refractive Index Distribution

Using a dual-beam interference microscope, a refractive indexdistribution W in the width direction was acquired for a transversecross-section of the core layer of the obtained optical waveguide. Theresults revealed that the refractive index distribution W had aplurality of low refractive index regions and high refractive indexregions, with the refractive index changing in a continuous manner.

Examples 14 and 15

With the exceptions of setting the monomer composition and content, andthe accumulated dose of ultraviolet radiation as shown in Table 3, andpreparing the photomask pattern so that the average width WCO of thecore portions and the average width WCL of the side cladding portionsexhibited the values shown in Table 3, optical waveguides were obtainedin the same manner as Example 13.

Example 16

(1) Production of Optical Waveguide

Using the optical waveguide-forming compositions used in Example 13, adie coater was used to perform multilayer extrusion molding onto apolyethersulfone (PES) film. As a result, a multilayer compact wasobtained by the extrusion of three layers, with the core layer-formingcomposition forming the center layer, and the cladding layer-formingcomposition forming the upper and lower layers. This multilayer compactwas placed in a dryer at 55° C. for 10 minutes to completely remove thesolvent. Subsequently, a photomask was pressed onto the multilayercompact, and selective irradiation was performed at 1,300 mJ/cm². Thephotomask was then removed, and the multilayer compact was heated in adryer at 150° C. for 1.5 hours. Following heating, the appearance of aclear waveguide pattern and the formation of core portions and sidecladding portions was confirmed. Subsequently, a sample having a lengthof 10 cm was cut from the obtained optical waveguide. The formed opticalwaveguide had eight core portions arranged in parallel. Further, thethickness of the entire optical waveguide was 100 μm.

(2) Evaluation of Refractive Index Distribution

Using a dual-beam interference microscope, a refractive indexdistribution W in the width direction was acquired for a transversecross-section of the core layer of the obtained optical waveguide. Theresults revealed that the refractive index distribution W had aplurality of low refractive index regions and high refractive indexregions, with the refractive index changing in a continuous manner.

On the other hand, for a transverse cross-section of the opticalwaveguide, the dual-beam interference microscope was also used toacquire a refractive index distribution T in the thickness directionalong a centerline that penetrates vertically along the center of thewidth of a core portion. The results revealed that the refractive indexdistribution T had a region in the central portion in which therefractive index changed continuously, and regions on both sides of thiscentral region in which the refractive index had a substantiallyconstant value that was lower than the refractive index of the centralregion. In other words, the refractive index distribution T in thethickness direction of the obtained optical waveguide was a so-calledgraded index type distribution.

Examples 17 and 18

With the exceptions of setting the monomer composition and content, andthe accumulated dose of ultraviolet radiation as shown in Table 3, andpreparing the photomask pattern so that the average width WCO of thecore portions and the average width WCL of the side cladding portionsexhibited the values shown in Table 3, optical waveguides of Examples 17and 18 were obtained in the same manner as Example 16.

Comparative Example 1

With the exceptions of not adding the CHOX to the core layer-formingcomposition and the cladding layer-forming composition, and altering theamount added of PI 2074 to 0.01 g, an optical waveguide of ComparativeExample 1 was obtained in the same manner as Example 13.

In the thus obtained optical waveguide, the refractive index of the coreportions was constant, and the refractive index of the side claddingportions was also constant, meaning the refractive index of the coreportions and the cladding portions was discontinuous. In other words,the refractive index distribution of the core layer of the obtainedoptical waveguide was a so-called step index (SI) type distribution.

Reference Examples 1 and 2

With the exception of altering the photomask pattern so that the averagewidth WCO of the core portions and the average width WCL of the sidecladding portions exhibited the values shown in Table 1, opticalwaveguides of Reference Examples 1 and 2 were obtained in the samemanner as Examples 1 and 2 respectively.

Reference Examples 3 and 4

With the exception altering the photomask pattern so that the averagewidth WCO of the core portions and the average width WCL of the sidecladding portions exhibited the values shown in Table 2, opticalwaveguides of Reference Examples 3 and 4 were obtained in the samemanner as Examples 9 and 10 respectively.

The production conditions for the optical waveguides obtained in each ofthe above examples, and each of the above comparative examples andreference examples are shown below in Tables 1, 2 and 3.

TABLE 1 (Epoxy-based Polymers) Core layer dimensions Core Claddingportion portion Core layer-forming composition Refractive index averageaverage Epoxy-based polymer Exposure distribution width width SofteningShore D Monomer Polymerization dose Width Thickness WCO WCL WCO/Composition point (° C.) hardness (phr) initiator (phr) (mJ/cm²)direction direction (μm) (μm) WCL Example 1 YP-50S 100 55 CelloxideSP-170 (1) 1,000 GI type SI type 45 20 2.25 2021P (25) Example 2 ↑ ↑ ↑ ↑↑ ↑ ↑ ↑ 35 200 0.18 Example 3 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 50 75 0.67 Example 4 ↑ ↑ ↑Celloxide ↑ ↑ ↑ ↑ 40 25 1.60 2021P (35) Example 5 Ogsol 115 70 Celloxide↑ 500 ↑ ↑ 45 20 2.25 EG 2021P (25) Example 6 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 35 200 0.18Example 7 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 50 75 0.67 Example 8 ↑ ↑ ↑ Celloxide ↑ ↑ ↑ ↑35 30 1.17 2021P (35) Reference YP-50S 100 55 Celloxide SP-170 (1) 1,000GI type ↑ 20 300 0.07 Example 1 2021P (25) Reference ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 504 12.50 Example 2 * In the table, ↑ means the cell has the same contentas the cell immediately above.

TABLE 2 (Acrylic-based Polymers) Core layer dimensions Core Claddingportion portion Core layer-forming composition Refractive index averageaverage Acrylic-based polymer Polymer- Exposure distribution width widthSoftening Shore D Monomer ization dose Width Thickness WCO WCL WCO/Composition point (° C.) hardness (phr) initiator (phr) (mJ/cm²)direction direction (μm) (μm) WCL Example 9 MMA + BzMA 95 60 CyclohexylIrgacure 800 GI type SI type 50 15 3.33 methacrylate 651 (1) (25)Example 10 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 40 85 0.47 Example 11 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 45 2050.22 Example 12 ↑ ↑ ↑ Cyclohexyl ↑ 1,000 ↑ ↑ 40 25 1.60 methacrylate(40) Reference MMA + BzMA 95 60 Cyclohexyl Irgacure 800 ↑ ↑ 20 300 0.07Example 3 methacrylate 651 (1) (25) Reference ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 80 4 20.00Example 4 * In the table, ↑ means the cell has the same content as thecell immediately above.

TABLE 3 (Polyolefin-based Polymers) Core layer dimensions Core Claddingportion portion Core layer-forming composition Refractive index averageaverage Polyolefin-based polymer Exposure distribution width widthSoftening Shore D Monomer Polymerization dose Width Thickness WCO WCLWCO/ Composition point (° C.) hardness (phr) initiator (phr) (mJ/cm²)direction direction (μm) (μm) WCL Example 13 Polymer #1 235 45 CHOX (20)PI 2074 (0.125) 1,300 GI type SI type 40 20 2.00 Example 14 ↑ ↑ ↑ ↑ ↑ ↑↑ ↑ 40 200 0.20 Example 15 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 50 75 0.67 Example 16 ↑ ↑ ↑ ↑↑ 1,300 ↑ GI type 50 75 0.67 Example 17 ↑ ↑ ↑ ↑ ↑ 1,500 ↑ ↑ 50 75 0.67Example 18 ↑ ↑ ↑ ↑ ↑ 500 ↑ ↑ 50 75 0.67 Comparative ↑ ↑ ↑ none PI 2074(0.1)  1,300 SI type SI type 45 80 0.56 Example 1 * In the table, ↑means the cell has the same content as the cell immediately above.2. Production of Optical Waveguide Having Refractive Index DistributionShown in FIG. 4

First, optical waveguides having linear core portions with therefractive index distribution illustrated in FIG. 4 were produced, andthese optical waveguides were then evaluated as described in Section 3below.

Examples 19 to 37, Comparative Example 2, and Reference Examples 5 to 10

With the exceptions of altering the production conditions as shown inTables 4, 5 and 6, altering the drying conditions when forming the corelayer to 50° C.×10 minutes in Examples 19 to 31 and Reference Examples 5to 8, and altering the drying conditions when forming the core layer to60° C.×15 minutes in Examples 32 to 37, Comparative Example 2 andReference Examples 9 and 19, optical waveguides were obtained in thesame manner as that described for Example 1. In Examples 35 to 37, theoptical waveguides were produced using the same method as Example 16.

TABLE 4 (Epoxy-based Polymers) Core layer dimensions Core Claddingportion portion Core layer-forming composition Refractive index averageaverage Epoxy-based polymer Exposure distribution width width SofteningShore D Monomer Polymerization dose Width Thickness WCO WCL WCO/Composition point (° C.) hardness (phr) initiator (phr) (mJ/cm²)direction direction (μm) (μm) WCL Example 19 YP-50S 100 55 CelloxideSP-170 (2) 800 W type SI type 45 20 2.25 2021P (25) Example 20 ↑ ↑ ↑ ↑ ↑↑ ↑ ↑ 40 200 0.20 Example 21 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 50 75 0.67 Example 22 ↑ ↑ ↑↑ ↑ 1,200 ↑ ↑ 30 12 2.50 Example 23 ↑ ↑ ↑ Celloxide ↑ 1,000 ↑ ↑ 35 152.33 2021P (35) Example 24 Ogsol EG 115 70 Celloxide ↑ 600 ↑ ↑ 50 301.67 2021P (25) Example 25 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 40 210 0.19 Example 26 ↑ ↑ ↑↑ ↑ ↑ ↑ ↑ 50 12 4.17 Example 27 ↑ ↑ ↑ Celloxide ↑ ↑ ↑ ↑ 40 85 0.47 2081(25) Reference YP-50S 100 55 Celloxide SP-170 (2) 500 ↑ ↑ 12 150 0.08Example 5 2021P (25) Reference ↑ ↑ ↑ ↑ ↑ 1,500 ↑ ↑ 50 4 12.50 Example6 * The ″W type″ refractive index distribution describes a refractiveindex distribution containing a region in which a second local maximumvalue, a local minimum value, a first local maximum value, a localminimum value, and a second local maximum value are arranged insequence. * In the table, ↑ means the cell has the same content as thecell immediately above.

TABLE 5 (Acrylic-based Polymers) Core layer dimensions Core Claddingportion portion Core layer-forming composition Refractive index averageaverage Acrylic-based polymer Polymer- Exposure distribution width widthSoftening Shore D Monomer ization dose Width Thickness WCO WCL WCO/Composition point (° C.) hardness (phr) initiator (phr) (mJ/cm²)direction direction (μm) (μm) WCL Example 28 MMA + BzMA 95 60 CyclohexylIrgacure 651 700 W type SI type 50 15 3.33 methacrylate (2) (25) Example29 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 40 85 0.47 Example 30 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 40 210 0.19Example 31 ↑ ↑ ↑ Cyclohexyl ↑ 1,200 ↑ ↑ 40 25 1.60 methacrylate (40)Reference MMA + BzMA 95 60 Cyclohexyl Irgacure 651 700 ↑ ↑ 12 150 0.08Example 7 methacrylate (2) (25) Reference ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 120 10 12.00Example 8 * The ″W type″ refractive index distribution describes arefractive index distribution containing a region in which a secondlocal maximum value, a local minimum value, a first local maximum value,a local minimum value, and a second local maximum value are arranged insequence. * In the table, ↑ means the cell has the same content as thecell immediately above.

TABLE 6 (Polyolefin-based Polymers) Core layer dimensions Core Claddingportion portion Core layer-forming composition Refractive index averageaverage Polyolefin-based polymer Exposure distribution width widthSoftening Shore D Monomer Polymerization dose Width Thickness WCO WCLWCO/ Composition point (° C.) hardness (phr) initiator (phr) (mJ/cm²)direction direction (μm) (μm) WCL Example 32 Polymer #1 235 45 CHOX PI2074 (0.25) 1,300 W type SI type 40 20 2.00 (20) Example 33 ↑ ↑ ↑ ↑ ↑ ↑↑ ↑ 40 200 0.20 Example 34 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 50 75 0.67 Example 35 ↑ ↑ ↑ ↑↑ 1,500 ↑ GI type 50 75 0.67 Example 36 ↑ 1 ↑ ↑ ↑ 1,800 ↑ ↑ 50 75 0.67Example 37 ↑ ↑ ↑ ↑ ↑ 700 ↑ ↑ 50 75 0.67 Comparative ↑ ↑ ↑ none PI 2074(0.1)  1,300 SI type SI type 40 20 2.00 Example 2 Reference ↑ ↑ ↑ ↑ ↑graded GI type ↑ 40 20 2.00 Example 9 exposure Reference ↑ ↑ ↑ ↑ ↑ ↑ Wtype ↑ 40 20 2.00 Example 10 * The ″W type″ refractive indexdistribution describes a refractive index distribution containing aregion in which a second local maximum value, a local minimum value, afirst local maximum value, a local minimum value, and a second localmaximum value are arranged in sequence. * In the table, ↑ means the cellhas the same content as the cell immediately above.3. Evaluation of Optical Waveguides3.1 Refractive Index Distribution of Optical Waveguide

For each optical waveguide, the refractive index distribution along thecenterline in the thickness direction of a transverse cross-section ofthe core layer was measured using a dual-beam interference microscope,thus obtaining a refractive index distribution in the width direction ofthe transverse cross-section of the core layer. In the obtainedrefractive index distribution, because the same refractive indexdistribution pattern repeated for each core portion, a portion of theobtained refractive index distribution was cut out and used as therefractive index distribution W. The refractive index distribution T wasobtained in a similar manner.

Among the various types of the refractive index distribution W, theshape of those distributions recorded as “GI type” in Tables 1, 2 and 3was similar to that illustrated in FIG. 3, including high refractiveindex regions WH containing a local maximum value Wm and low refractiveindex regions WL arranged alternately.

Further, among the various types of the refractive index distribution W,the shape of those distributions recorded as “W type” in Tables 4, 5 and6 was similar to that illustrated in FIG. 4, including four localminimum values and five local maximum values arranged alternately. Fromthis W type refractive index distribution W, each of the local minimumvalues Ws1, Ws2, Ws3 and Ws4, and each of the local maximum values Wm1,Wm2, Wm3, Wm4 and Wm5 were determined, and the average refractive indexWA in the cladding portions was also determined. In the refractive indexdistribution W in the width direction of the optical waveguide obtainedin each of the examples and each of the reference examples, therefractive index changed continuously across the entire distribution.

Further, in this W type refractive index distribution W, the width a[μm] of the portions in the vicinity of the local maximum values Wm2 andWm4 in which the refractive index has a value equal to or greater thanthe average refractive index WA, and the width b [μm] of the portions inthe vicinity of the local minimum values Ws1, Ws2, Ws3 and Ws4 in whichthe refractive index has a value less than the average refractive indexWA were also measured.

Furthermore, in each of the optical waveguides, the maximum variation inthe refractive index in the tailing portions was within a range from0.008 to 0.025. Moreover, the maximum value for the refractive index ineach of the intersection portions was higher than the local maximumvalue Wm, with the difference being within a range from 0.003 to 0.015.

The above measurement results are shown in Tables 7 to 13.

TABLE 7 Table 7 (Epoxy-based polymers) Refractive index distribution WWL Wm Wm − WL Example 1 1.535 1.548 0.013 Example 2 1.540 1.549 0.009Example 3 1.538 1.548 0.010 Example 4 1.530 1.547 0.017 Example 5 1.5801.601 0.021 Example 6 1.585 1.602 0.017 Example 7 1.582 1.600 0.018Example 8 1.575 1.602 0.027 Reference Example 1 1.542 1.550 0.008Reference Example 2 1.545 1.549 0.004

TABLE 8 Table 8 (Acrylic-based polymers) Refractive index distribution WWL Wm Wm − WL Example 9 1.502 1.513 0.011 Example 10 1.503 1.515 0.012Example 11 1.506 1.514 0.008 Example 12 1.498 1.513 0.015 ReferenceExample 3 1.505 1.512 0.007 Reference Example 4 1.508 1.512 0.004

TABLE 9 Table 9 (Polyolefin-based polymers) Refractive indexdistribution W WL Wm Wm − WL Example 13 1.543 1.557 0.014 Example 141.546 1.556 0.010 Example 15 1.544 1.556 0.012 Example 16 1.547 1.5570.010 Example 17 1.544 1.557 0.013 Example 18 1.549 1.557 0.008Comparative Example 1 Step index type

TABLE 10 Table 10 (Polyolefin-based polymers) Refractive indexdistribution T TA Tm Tm − TA Example 16 1.537 1.557 0.020 Example 171.537 1.557 0.020 Example 18 1.537 1.557 0.020

TABLE 11 (Epoxy-based polymers) Parameters of refractive indexdistribution W in width direction Average refractive (WA − Ws1)/ (Wm1 −Ws1)/ index (Wm2 − Ws1) × (Wm2 − Ws1) × a Wm1 Ws1 Wm2 Ws2 Wm3 Ws3 Wm4Ws4 Wm5 WA 100 100 Wm2 − Ws1 [μm] b Example 1.538 1.534 1.550 1.5341.538 1.534 1.550 1.534 1.538 1.5360 12.5 25.0 0.016 38 0.30a 19 Example1.540 1.538 1.551 1.538 1.540 1.538 1.551 1.538 1.540 1.5390 7.7 15.40.013 30 0.92a 20 Example 1.540 1.536 1.551 1.536 1.540 1.536 1.5511.536 1.540 1.5380 13.3 26.7 0.015 42 0.55a 21 Example 1.537 1.534 1.5491.534 1.537 1.534 1.549 1.534 1.537 1.5355 10.0 20.0 0.015 25 0.28a 22Example 1.533 1.525 1.549 1.525 1.533 1.525 1.549 1.525 1.533 1.529016.7 33.3 0.024 28 0.42a 23 Example 1.583 1.574 1.600 1.574 1.583 1.5741.600 1.574 1.583 1.5785 17.3 34.6 0.026 43 0.44a 24 Example 1.584 1.5781.602 1.578 1.584 1.578 1.602 1.578 1.584 1.5810 12.5 25.0 0.024 351.1a  25 Example 1.586 1.575 1.601 1.575 1.586 1.575 1.601 1.575 1.5861.5805 21.2 42.3 0.026 42 0.28a 26 Example 1.583 1.580 1.598 1.580 1.5831.580 1.598 1.580 1.583 1.5815 8.3 16.7 0.018 37 0.79a 27 Reference1.541 1.540 1.545 1.540 1.541 1.540 1.545 1.540 1.541 1.5405 10.0 20.00.005 9 1.5a  Example  5 Reference 1.546 1.545 1.549 1.545 1.546 1.5451.549 1.545 1.546 1.5455 12.5 25.0 0.004 48 0.05a Example  6

TABLE 12 (Acrylic-based polymers) Parameters of refractive indexdistribution W in width direction Average refractive (WA − Ws1)/ (Wm1 −Ws1)/ index (Wm2 − Ws1) × (Wm2 − Ws1) × a Wm1 Ws1 Wm2 Ws2 Wm3 Ws3 Wm4Ws4 Wm5 WA 100 100 Wm2 − Ws1 [μm] b Example 1.503 1.498 1.514 1.4981.503 1.498 1.514 1.498 1.503 1.5005 15.6 31.2 0.016 44 0.25a 28 Example1.502 1.500 1.515 1.500 1.502 1.500 1.515 1.500 1.502 1.5010 6.7 13.30.015 36 0.62a 29 Example 1.502 1.501 1.514 1.501 1.502 1.501 1.5141.501 1.502 1.5015 3.8 7.7 0.013 35 0.85a 30 Example 1.498 1.492 1.5131.492 1.498 1.492 1.513 1.492 1.498 1.4950 14.3 28.6 0.021 35 0.45a 31Reference 1.505 1.503 1.511 1.503 1.505 1.503 1.511 1.503 1.505 1.504012.5 25.0 0.008 11 1.5a  Example  7 Reference 1.505 1.504 1.513 1.5041.505 1.504 1.513 1.504 1.505 1.5045 5.6 11.1 0.009 75 0.08a Example  8

TABLE 13 (Polyolefin-based polymers) Parameters of refractive indexdistribution W in width direction Average refractive (WA − Ws1)/ (Wm1 −Ws1)/ index (Wm2 − Ws1) × (Wm2 − Ws1) × a Wm1 Ws1 Wm2 Ws2 Wm3 Ws3 Wm4Ws4 Wm5 WA 100 100 Wm2 − Ws1 [μm] b Example 1.545 1.541 1.557 1.5411.545 1.541 1.557 1.541 1.545 1.5430 12.5 25.0 0.016 35 0.15a 32 Example1.548 1.544 1.556 1.544 1.548 1.544 1.556 1.544 1.548 1.5460 16.7 33.30.012 38 0.82a 33 Example 1.546 1.542 1.556 1.542 1.546 1.542 1.5561.542 1.546 1.5440 14.3 28.6 0.014 45 0.3a  34 Example 1.546 1.543 1.5571.543 1.546 1.543 1.557 1.543 1.546 1.5445 10.7 21.4 0.014 42 0.32a 35Example 1.544 1.541 1.557 1.541 1.544 1.541 1.557 1.541 1.544 1.5425 9.418.8 0.016 38 0.45a 36 Example 1.549 1.546 1.557 1.546 1.549 1.546 1.5571.546 1.549 1.5475 13.6 27.3 0.011 44 0.28a 37 Compar- The refractiveindex distribution W is a step index type distribution ative Example  2Reference The refractive index distribution W is a graded index typedistribution Example  9 Reference The refractive index distribution W isdiscontinuous Example 10

The refractive index distribution W in the width direction of theoptical waveguides obtained in Comparative examples 1 and 2 were stepindex type distributions.

3.2 Transmission Loss of Optical Waveguide

Light emitted from an 850 nm VCSEL (surface emitting laser) wasintroduced through an optical fiber of diameter 50 μm into the opticalwaveguide obtained in each example and each comparative example, theoutput light was received by an optical fiber of diameter 200 μm, andthe intensity of the light was measured. Measurement of the transmissionloss was performed using the cutback method. When the measured valueswere plotted with the longitudinal direction of the optical waveguidealong the horizontal axis and the insertion loss along the verticalaxis, the measured values fell along a straight line. The transmissionloss was calculated from the slope of this straight line. The resultsare shown below in Tables 14 to 19.

3.3 Retention of Pulse Signal Waveform

For each of the obtained optical waveguides, a pulse signal having apulse width of 1 ns was input into the optical waveguide from a laserpulse source, and the pulse width of the output light was measured.

For this measured pulse width of the output light, a relative value wascalculated, either with the measured value for the optical waveguideobtained in Comparative Example 1 denoted as 1 in Tables 14 to 16, orwith the measured value for the optical waveguide obtained inComparative Example 2 denoted as 1 in Tables 17 to 19, and this relativevalue was evaluated against the evaluation criteria shown below. Theresults are shown below in Tables 14 to 19.

<Evaluation Criteria for Pulse Width>

A: relative value of the pulse width is less than 0.5

B: relative value of the pulse width is at least 0.5 but less than 0.8

C: relative value of the pulse width is at least 0.8 but less than 1

D: relative value of the pulse width is 1 or greater

The results of the above evaluations 3.2 and 3.3 are shown in Tables 14to 19.

TABLE 14 (Epoxy-based polymers) Amount of Evaluation resultsIntersection portion loss interference Transmission Pulse [dB/cross]light loss [dB/cm] width 30° 60° 90° (relative value) Example 1 0.04 AExample A 0.047 0.021 0.016 0.75 Example 2 0.11 C Example B 0.053 0.0260.020 0.87 Example 3 0.10 B Example C 0.051 0.028 0.020 0.84 Example 40.05 B Example D 0.052 0.026 0.018 0.81 Example 5 0.05 A Example E 0.0470.024 0.017 0.78 Example 6 0.07 B Example F 0.051 0.025 0.019 0.80Example 7 0.10 B Example G 0.053 0.026 0.019 0.85 Example 8 0.04 AExample H 0.046 0.025 0.017 0.76 Reference 0.15 C Reference 0.060 0.0330.028 0.95 Example 1 Example A Reference 0.35 D Reference 0.083 0.0390.042 0.98 Example 2 Example B

TABLE 15 (Acrylic-based polymers) Amount of Evaluation resultsIntersection portion loss interference Transmission Pulse [dB/cross]light loss [dB/cm] width 30° 60° 90° (relative value) Example 9 0.05 AExample I 0.047 0.025 0.017 0.76 Example 10 0.08 B Example J 0.052 0.0270.019 0.82 Example 11 0.07 C Example K 0.051 0.028 0.019 0.87 Example 120.06 A Example L 0.049 0.024 0.018 0.77 Reference 0.17 C Reference 0.0700.038 0.029 0.95 Example 3 Example C Reference 0.55 D Reference 0.2270.072 0.039 0.97 Example 4 Example D

TABLE 16 (Polyolefin-based polymers) Amount of Evaluation resultsIntersection portion loss interference Transmission Pulse [dB/cross]light loss [dB/cm] width 30° 60° 90° (relative value) Example 13 0.05 AExample M 0.051 0.023 0.017 0.72 Example 14 0.08 B Example N 0.053 0.0260.018 0.83 Example 15 0.07 C Example O 0.062 0.027 0.019 0.88 Example 160.04 A Example P 0.013 0.009 0.006 0.70 Example 17 0.03 A Example Q0.012 0.009 0.005 0.68 Example 18 0.05 A Example R 0.033 0.013 0.0070.73 Comparative 0.24 — Comparative 0.123 0.066 0.039 1 Example 1Example A

TABLE 17 (Epoxy-based polymers) Amount of Evaluation resultsIntersection portion loss interference Transmission Pulse [dB/cross]light loss [dB/cm] width 30° 60° 90° (relative value) Example 19 0.04 AExample S 0.046 0.021 0.017 0.72 Example 20 0.08 C Example T 0.051 0.0260.019 0.91 Example 21 0.06 B Example U 0.050 0.027 0.018 0.79 Example 220.05 A Example V 0.049 0.021 0.017 0.73 Example 23 0.07 B Example W0.051 0.025 0.018 0.81 Example 24 0.05 A Example X 0.047 0.023 0.0170.74 Example 25 0.08 C Example Y 0.051 0.024 0.019 0.87 Example 26 0.06C Example Z 0.052 0.023 0.018 0.86 Example 27 0.05 B Example a 0.0460.024 0.017 0.83 Reference 0.23 C Reference 0.058 0.031 0.027 0.94Example 5 Example E Reference 0.30 C Reference 0.079 0.046 0.041 0.96Example 6 Example F

TABLE 18 (Acrylic-based polymers) Amount of Evaluation resultsIntersection portion loss interference Transmission Pulse [dB/cross]light loss [dB/cm] width 30° 60° 90° (relative value) Example 28 0.05 AExample b 0.047 0.024 0.017 0.74 Example 29 0.07 B Example c 0.050 0.0270.018 0.82 Example 30 0.09 C Example d 0.052 0.037 0.019 0.87 Example 310.06 A Example e 0.050 0.023 0.017 0.77 Reference 0.23 C Reference 0.0670.037 0.031 0.97 Example 7 Example G Reference 0.33 C Reference 0.1530.069 0.036 0.99 Example 8 Example H

TABLE 19 (Polyolefin-based polymers) Amount of Evaluation resultsIntersection portion interference Transmission Pulse loss [dB/cross]light loss [dB/cm] width 30° 60° 90° (relative value) Example 33 0.04 AExample f 0.049 0.023 0.016 0.71 Example 34 0.06 C Example g 0.052 0.0280.019 0.84 Example 35 0.04 B Example h 0.053 0.026 0.018 0.79 Example 360.03 A Example i 0.012 0.009 0.005 0.67 Example 37 0.03 A Example j0.011 0.009 0.005 0.66 Comparative 0.05 A Example k 0.029 0.011 0.0060.69 Example 2 0.21 — Comparative 0.123 0.066 0.039 1 Reference ExampleB Example 9 0.12 C Reference 0.100 0.043 0.026 0.95 Reference Example IExample 10 0.10 C Reference 0.086 0.039 0.022 0.97 Example J

As is evident from Tables 14 to 19, in the optical waveguide obtained ineach of the examples, transmission loss and rounding of the pulse signalwere able to be suppressed compared with the optical waveguides obtainedin each of the comparative examples.

In the case of the core layer-forming composition used in ComparativeExample 1, for which the photo-bleaching phenomenon occurs, because theamount of change in the refractive index can be adjusted in accordancewith the amount of irradiated light, a test was conducted utilizing thisproperty, in which a refractive index distribution W was formed using aphotomask that was set so that the accumulated irradiation dosegradually changed. When the refractive index distribution of the thusobtained optical waveguide was evaluated in the same manner as thatdescribed above, although high refractive index regions and lowrefractive index regions were confirmed, the change in refractive indexwas not as continuous as that observed in the examples. Further, thetransmission loss in the obtained optical waveguide was larger than thatobserved in any of the examples, and retention of the pulse signalwaveform was also poor.

4. Production of Optical Waveguides Having Intersection Portions

Subsequently, using the same conditions as each of the above examples,comparative examples and reference examples, optical waveguides havingintersection portions were produced in the manner described below.

Example A

With the exception of using a photomask corresponding with the patternof an optical waveguide having intersection portions as the photomaskused during preparation of the core layer, an optical waveguide wasproduced in the same manner as that described for Example 1, thusproducing an optical waveguide having intersection portions. In theproduction of the optical waveguide, three types of optical waveguideswere produced in which the intersection angle at each intersectionportion was 30°, 60° and 90° respectively.

Examples B to Z, and Examples a to k, Comparative Examples A and B, andReference Examples A to J

With the exception of using a photomask corresponding with the patternof an optical waveguide having intersection portions as the photomaskused during preparation of the core layer, optical waveguides wereproduced in the same manner as that described for Examples 2 to 37,Comparative Examples 1 and 2, and Reference Examples 1 to 10, thusproducing optical waveguides having intersection portions. In theproduction of each optical waveguide, three types of optical waveguideswere produced in which the intersection angle at each intersectionportion was 30°, 60° and 90° respectively.

5. Evaluation of Optical Waveguide Having Intersection Portions

Subsequently, for each of the obtained optical waveguides havingintersection portions, the insertion loss between the two end portionswas measured. The results revealed that the value for the insertion lossexhibited the same tendency as the aforementioned transmission loss. Inother words, whereas the insertion loss was satisfactorily small in theoptical waveguides having intersection portions obtained in each of theexamples, the insertion loss was comparatively large in the opticalwaveguides having intersection portions obtained in each of thecomparative examples. Further, it was found that optical waveguides forwhich the transmission loss measured in Section 2 was small alsoexhibited small amounts of interference of the signal light.

Furthermore, when the transmission loss in the intersection portions wascalculated, it was clear that the optical waveguides having intersectionportions obtained in each of the examples exhibited less transmissionloss than the optical waveguides having intersection portion obtained ineach of the comparative examples. The calculated transmission loss perintersection portion is shown in Tables 14 to 19. When the angle ofintersection was 90°, the transmission loss was 0.02 dB or less in allof the examples.

Further, the method used for calculating the transmission loss in theintersection portion involved preparing a plurality of samples havingdifferent numbers of intersection portions, and then calculating thetransmission loss per intersection portion by comparing the insertionloss for these samples.

Furthermore, the amount of signal light interfering with the coreportion intersecting the core portion that represents the measurementtarget (hereafter referred to as the “amount of interference light”) wasalso measured. For each of these measured amounts of interference light,a relative value was calculated, either with the measured value for theoptical waveguide obtained in Comparative Example 1 denoted as 1 inTables 14 to 16, or with the measured value for the optical waveguideobtained in Comparative Example 2 denoted as 1 in Tables 17 to 19. Theserelative values are shown in Tables 14 to 19.

The results confirmed that by optimizing the refractive indexdistribution W, the amount of interfering signal light could be reduced.

Based on the above results, it was evident that in an optical waveguidehaving core portions in which the refractive index distribution is acontinuous distribution that satisfies specific conditions, loss andinterference could be suppressed.

INDUSTRIAL APPLICABILITY

The present invention provides an optical waveguide in which coreportions can be intersected without an accompanying increase inthickness, and the core portions can be formed at high density, and alsoprovides an optical wiring component and an optical waveguide modulewhich include the optical waveguide and can simplify optical wiring andcontribute to space saving, and an electronic device which can bereadily miniaturized.

DESCRIPTION OF THE REFERENCE SIGNS

-   1: Optical waveguide-   10: Optical wiring component-   101: Connector-   11, 12, 121, 122: Cladding layer-   13, 131, 132: Core layer-   14: Core portion-   140: Core group-   141, 142, 143, 144, 145, 146: Core portion-   147, 148: Intersection portion-   15: Side cladding portion-   151, 152, 153, 154, 155, 156: Side cladding portion-   2: Support film-   3: Cover film-   901: Optical waveguide-forming composition (first composition)-   902: Optical waveguide-forming composition (second composition)-   910: Layer-   914: Multilayer compact-   914 a: First molded layer-   914 b: Second molded layer-   914 c: Third molded layer-   914 d: Fourth molded layer-   914 e: Fifth molded layer-   914 f: Sixth molded layer-   914 g: Seventh molded layer-   914 h: Eighth molded layer-   914 i: Ninth molded layer-   915: Polymer-   920: Additive-   930: Active radiation-   935: Mask (masking)-   9351: Opening (window)-   925: Irradiated region-   9251, 9252, 9253: Irradiated region-   940: Non-irradiated region-   9403: Non-irradiated region-   951: Support substrate-   C1, C2, C2′: Centerline-   W: Refractive index distribution-   P2: Intensity distribution-   T, T′: Refractive index distribution-   H: High refractive index portion-   L: Low refractive index portion

The invention claimed is:
 1. An optical waveguide, comprising: a core layer having a plurality of core portions and a plurality of side cladding portions adjoining the plurality of core portions such that the plurality of core portions comprises a plurality of core groups mutually intersecting on a same plane, each of the core groups including at least a pair of the core portions formed in parallel, wherein the core layer has a transverse cross-section such that a refractive index distribution W in a width direction of the core layer has a region having first and second local minimum values, a local maximum value, and first and second local secondary maximum values smaller than the local maximum value, the first and second local minimum values, the local maximum value and the first and second local secondary maximum values are in an order of the first local secondary maximum value, the first local minimum value, the local maximum value, the second local minimum value and the second local secondary maximum value, a section in the region between the first and second local minimum values including the local maximum value corresponds to the core portion, sections in the region including each of the first and second local secondary maximum values correspond to the side cladding portions, each of the first and second local minimum values has a value smaller than an average refractive index in the cladding portions, and the refractive index distribution W varies continuously in the width direction of the core layer.
 2. The optical waveguide according to claim 1, wherein the core layer comprises a polymer and a refractive index modifier having a different refractive index from a refractive index of the polymer, and the refractive index distribution W is formed in correspondence with a concentration of the refractive index modifier.
 3. The optical waveguide according to claim 2, wherein the core layer comprises the polymer and a photopolymerizable monomer having a different refractive index from the polymer and dispersed within the polymer, the refractive index distribution W is formed by a process comprising irradiating light partially onto the core layer such that the photopolymerizable monomer migrates and is unevenly distributed to generate a variation of refractive index within the polymer layer.
 4. The optical waveguide according to claim 1, wherein, when two core portions mutually intersect at an angle of intersection of optical axes of 90°, a transmission loss in the intersection of the two core portions is not more than 0.02 dB.
 5. The optical waveguide according to claim 1, wherein the core portions have a width of from 10 to 200 μm.
 6. An optical wiring component, comprising: the optical waveguide according to claim 1; and a plurality of connectors positioned at ends of the core groups of the optical waveguide.
 7. The optical wiring component according to claim 6, wherein the optical waveguide has an optical path conversion portion formed on each of the core portions or on an extended line of each of the core portions, and the optical path conversion portion converts the optical path of each of the core portions.
 8. An optical waveguide module, comprising: the optical waveguide according to claim 1; and a plurality of light receiving and emitting elements, wherein the light receiving and emitting elements are positioned on one surface of the optical waveguide and are optically connected to the core portions.
 9. An electronic device, comprising: the optical waveguide according to claim
 1. 10. The optical waveguide according to claim 1, further comprising: a plurality of cladding layers, wherein the cladding layers are laminated on both surfaces of the core layer, respectively, the optical waveguide has a transverse cross-section such that a refractive index distribution T in a thickness direction is substantially constant in a region corresponding to the core portion and regions corresponding to the cladding layers, and the refractive index distribution T varies discontinuously at interfaces between the core portion and the cladding layers.
 11. The optical waveguide according to claim 1, further comprising: a plurality of cladding layers, wherein the cladding layers are laminated on both surfaces of the core layer, respectively, the optical waveguide has a transverse cross-section such that a refractive index distribution T in a thickness direction comprises a region corresponding to the core portion and regions corresponding to the cladding layers, the refractive index distribution T varies continuously in the region corresponding to the core portion, the refractive index distribution T is substantially constant in the regions corresponding to the cladding layers, and the refractive index distribution T varies discontinuously at interfaces between the core portion and the cladding layers.
 12. The optical waveguide according to claim 1, further comprising: a plurality of cladding layers, wherein the cladding layers are laminated on both surfaces of the core layer, respectively, the optical waveguide has a transverse cross-section such that a refractive index distribution T in a thickness direction has a local maximum value, first portions and second portions, the refractive index distribution T in the first portions decreases continuously from the local maximum value toward the cladding layers, the second portions are positioned on upper and lower surface sides of the first portions, a refractive index in the second portions is substantially constant, and a region including the local maximum value and the first portions corresponds to the core portion, and regions including each of the second portions correspond to the cladding layers.
 13. The optical waveguide according to claim 1, further comprising: a plurality of cladding layers, wherein the cladding layers are laminated on both surfaces of the core layer, respectively, the optical waveguide has a transverse cross section such that a refractive index distribution T in a thickness direction has a region having first and second local minimum values, a local maximum value, and first and second local secondary maximum values smaller than the local maximum value, the first and second local minimum values, the local maximum value, and the first and second local secondary maximum values are arranged in an order of the first local secondary maximum value, the first local minimum value, the local maximum value, the second local minimum value and the second local secondary maximum value, a section in the region between the first and second local minimum values including the local maximum value corresponds to the core layer, sections in the region including each of the first and second local maximum values correspond to the cladding layers, each of the first and second local minimum values has a value smaller than an average refractive index in the cladding layers, and the entire refractive index distribution T varies continuously.
 14. The optical waveguide according to claim 1, wherein a thickness of the core layer is about 1 to 200 μm, an average width of the side cladding portions is within a range from 5 to 250 μm, a ratio of an average width of the core portions to an average width of the side cladding portions is within a range from 0.1 to 10, an average thickness of the cladding layers is from 0.01 to 7 times of an average thickness of the core layer, or in a transverse cross-section of the core layer, when a denotes a width of a portion of the core layer in which a refractive index is continuously equal to or greater than an average refractive index of the side cladding portions, and b denotes a width of a portion of the core layer in which a refractive index is continuously less than the average refractive index of the side cladding portions, b is within a range from 0.01a to 1.2a.
 15. The optical waveguide according to Claim 1, wherein, in the refractive index distribution W, a difference between an average refractive index of the first and second local minimum values and an average refractive index of the side cladding portions is from 3 to 80% of a difference between the average refractive index of the first and second local minimum values and an average refractive index of the local maximum value, a difference between the average refractive index of the first and second local minimum values and an average refractive index of the first and second local secondary maximum values is from 6 to 90% of a difference between the average refractive index of the first and second local minimum values and the average refractive index of the local maximum value, or the difference between the average refractive index of the first and second local minimum values and the average refractive index of the local maximum value is from 0.005 to 0.07.
 16. The optical waveguide according to claim 10, wherein, in the refractive index distribution T, when a denotes a width of a portion in which a refractive index is equal to or greater than an average refractive index of the cladding layers, and b denotes a width of a portion in which a refractive index is less than the average refractive index of the cladding layers, b is within a range from 0.01a to 1.2a.
 17. The optical waveguide according to claim 11, wherein, in the refractive index distribution T, when a denotes a width of a portion in which a refractive index is equal to or greater than an average refractive index of the cladding layers, and b denotes a width of a portion in which a refractive index is less than the average refractive index of the cladding layers, b is within a range from 0.01a to 1.2a.
 18. The optical waveguide according to claim 12, wherein, in the refractive index distribution T, when a denotes a width of a portion in which a refractive index is equal to or greater than an average refractive index of the cladding layers, and b denotes a width of a portion in which a refractive index is less than the average refractive index of the cladding layers, b is within a range from 0.01 a to 1.2a.
 19. The optical waveguide according to claim 13, wherein, in the refractive index distribution T, when a denotes a width of a portion in which a refractive index is equal to or greater than an average refractive index of the cladding layers, and b denotes a width of a portion in which a refractive index is less than the average refractive index of the cladding layers, b is within a range from 0.01a to 1.2a, a difference between an average refractive index of the first and second local minimum values and an average refractive index of the cladding layers is from 3 to 80% of a difference between the average refractive index of the first and second local minimum values and the local maximum value within the core portion, or a difference between the average refractive index of the first and second local minimum values and the average refractive index of the local maximum value within the core portion is from 0.005 to 0.07. 